This is a major change on how we represent nested name qualifications in
the AST.

* The nested name specifier itself and how it's stored is changed. The
prefixes for types are handled within the type hierarchy, which makes
canonicalization for them super cheap, no memory allocation required.
Also translating a type into nested name specifier form becomes a no-op.
An identifier is stored as a DependentNameType. The nested name
specifier gains a lightweight handle class, to be used instead of
passing around pointers, which is similar to what is implemented for
TemplateName. There is still one free bit available, and this handle can
be used within a PointerUnion and PointerIntPair, which should keep
bit-packing aficionados happy.
* The ElaboratedType node is removed, all type nodes in which it could
previously apply to can now store the elaborated keyword and name
qualifier, tail allocating when present.
* TagTypes can now point to the exact declaration found when producing
these, as opposed to the previous situation of there only existing one
TagType per entity. This increases the amount of type sugar retained,
and can have several applications, for example in tracking module
ownership, and other tools which care about source file origins, such as
IWYU. These TagTypes are lazily allocated, in order to limit the
increase in AST size.

This patch offers a great performance benefit.

It greatly improves compilation time for
[stdexec](https://github.com/NVIDIA/stdexec). For one datapoint, for
`test_on2.cpp` in that project, which is the slowest compiling test,
this patch improves `-c` compilation time by about 7.2%, with the
`-fsyntax-only` improvement being at ~12%.

This has great results on compile-time-tracker as well:

![image](https://github.com/user-attachments/assets/700dce98-2cab-4aa8-97d1-b038c0bee831)

This patch also further enables other optimziations in the future, and
will reduce the performance impact of template specialization resugaring
when that lands.

It has some other miscelaneous drive-by fixes.

About the review: Yes the patch is huge, sorry about that. Part of the
reason is that I started by the nested name specifier part, before the
ElaboratedType part, but that had a huge performance downside, as
ElaboratedType is a big performance hog. I didn't have the steam to go
back and change the patch after the fact.

There is also a lot of internal API changes, and it made sense to remove
ElaboratedType in one go, versus removing it from one type at a time, as
that would present much more churn to the users. Also, the nested name
specifier having a different API avoids missing changes related to how
prefixes work now, which could make existing code compile but not work.

How to review: The important changes are all in
`clang/include/clang/AST` and `clang/lib/AST`, with also important
changes in `clang/lib/Sema/TreeTransform.h`.

The rest and bulk of the changes are mostly consequences of the changes
in API.

PS: TagType::getDecl is renamed to `getOriginalDecl` in this patch, just
for easier to rebasing. I plan to rename it back after this lands.

Fixes #136624
Fixes https://github.com/llvm/llvm-project/issues/43179
Fixes https://github.com/llvm/llvm-project/issues/68670
Fixes https://github.com/llvm/llvm-project/issues/92757

Split out from https://github.com/llvm/llvm-project/pull/148877

This patch prepares `TypeSystemClang` APIs to take `AsmLabel`s which
concatenated strings (hence `std::string`) instead of a plain `const
char*`.

The `DWARFASTParserClang` reads enum values as `int64_t`s regardless of
the enumerators signedness. Then we pass it to
`AddEnumerationValueToEnumerationType` and only then do we create an
`APSInt` from it. However, there are other places where we read/pass
around the enum value as unsigned. This patch makes sure we consistently
use the same integer type for the enum value and let `APSInt` take care
of signedness. This shouldn't have any observable effect.

While sifting through this part of the code I noticed that when we parse
C++ methods, `DWARFASTParserClang` creates two sets of `ParmVarDecls`,
one in `ParseChildParameters` and once in `AddMethodToCXXRecordType`.
The former is unused when we're dealing with methods. Moreover, the
`ParmVarDecls` we created in `ParseChildParameters` were created with an
incorrect `clang::DeclContext` (namely the DeclContext of the function,
and not the function itself). In Clang, there's
`ParmVarDecl::setOwningFunction` to adjust the DeclContext of a
parameter if the parameter was created before the FunctionDecl. But we
never used it.

This patch removes the `ParmVarDecl` creation from
`ParseChildParameters` and instead creates a
`TypeSystemClang::CreateParameterDeclarations` that ensures we set the
DeclContext correctly.

Note there is one differences in how `ParmVarDecl`s would be created
now: we won't set a ClangASTMetadata entry for any of the parameters. I
don't think this was ever actually useful for parameter DIEs anyway.

This wasn't causing any concrete issues (that I know of), but was quite
surprising. And this way of setting the parameters seems easier to
reason about (in my opinion).

This is a follow-up to https://github.com/llvm/llvm-project/pull/102161
where we changed the `GetMetadata`/`SetMetadata` APIs to pass
`ClangASTMetadata` by-value, instead of `ClangASTMetadata *`, which
wasn't a very friendly API.

This patch continues from there and changes
`CreateRecordType`/`CreateObjCClass` to take the metadata by-value as
well.

As a drive-by change, I also changed `DelayedAddObjCClassProperty` to
store the metadata by-value, instead of in a `std::unique_ptr`, which
AFAICT, was done solely due to the TypeSystemClang APIs taking the
metadata by pointer. This meant we could also get rid of the
user-provided copy constructors.

Note that StringRef::equals has been deprecated in favor of
operator==(StringRef, StringRef).

This patch converts TagTypeKind into scoped enum. Among other benefits,
this allows us to forward-declare it where necessary.

This method just takes its ConstString parameter and gets a StringRef
out of it. Let's just pass in a StringRef directly.

This also cleans up some logic in the callers to be a little easier to
read and to avoid unnecessary ConstString creation.

Differential Revision: https://reviews.llvm.org/D153054

Ensure that the variant returned by `member->getValue()` has a value and
is not `Empty`.  Failure to do so will trigger an assertion failure in
`llvm::pdb::Variant::getBitWidth()`.  This can occur when the `static`
member is a forward declaration.

Differential Revision: https://reviews.llvm.org/D146536
Reviewed By: sgraenitz