The `ClangDeclVendor` used to contain more Clang-specific code than it
does nowadays. But at this point, all it does is wrap the
`DeclVendor::FindDecls` call and copy the resulting decls into
`std::vector<clang::NamedDecl*>`. I.e., it converts the generic
`CompilerDecl`s to `clang::NamedDecl*`s.

In my opinion at this point it doesn't do enough to justify making it
part of the `DeclVendor` hierarchy.

This patch removes the `ClangDeclVendor` and instead does the conversion
at callsite.

This rename was made as part of
https://github.com/llvm/llvm-project/pull/147835 in order to ease
rebasing the PR, and give a nice window for other patches to get rebased
as well.

It has been a while already, so lets go ahead and rename it back.

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*`.

I've been skimming this code while investigating a bug around module
lookup and this looked like something we could clean up. We don't need
to be carrying around state inside of `NameSearchContext` to tell us to
look into modules. We can signal this via a boolean return from
`LookupFunction`.

Add an overloaded `GetTypeSystem` to specify the expected type system subclass. Changes code from  `GetTypeSystem().dyn_cast_or_null<TypeSystemClang>()` to `GetTypeSystem<TypeSystemClang>()`.

The original PR is #132274.

Co-authored-by: @bulbazord Alex Langford

Reverts llvm/llvm-project#132274

Broke a test on LLDB Widows on Arm:
https://lab.llvm.org/buildbot/#/builders/141/builds/7726
```
FAIL: test_dwarf (lldbsuite.test.lldbtest.TestExternCSymbols.test_dwarf)
<...>
    self.assertTrue(self.res.Succeeded(), msg + output)

AssertionError: False is not true : Command 'expression -- foo()' did not return successfully

Error output:

error: Couldn't look up symbols:

  int foo(void)

Hint: The expression tried to call a function that is not present in the target, perhaps because it was optimized out by the compiler.
```

This patch addresses the issue #129543.
After this patch the size of lldb-server is reduced by 9MB.

Co-authored-by: @bulbazord Alex Langford