PDB doesn't include the typedefs for types, so all types use their full
name. For `std::string` and friends, this means they show up as
`std::basic_string<char, std::char_traits<char>, std::allocator<char>>`.

This PR updates the `std::{,w,u8,u16,u32}string(_view)` tests to account
for this and runs them with PDB.

This came up in https://github.com/llvm/llvm-project/issues/155691.

For `std::basic_string` our formatter matching logic required the
allocator template parameter to be a `std::allocator`. There is no
compelling reason (that I know of) why this would be required for us to
apply the existing formatter to the string. We don't check the
`allocator` parameter for other STL containers either. This meant that
`std::string` that used custom allocators wouldn't be formatted. This
patch relaxes the regex for `basic_string`.

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

These were mistakenly changed during https://github.com/llvm/llvm-project/pull/147525

Now that most STL formatter tests have been moved to `generic`. Do the
same for the MSVC tests (which are currently just for `std::string`).
The `std::string` test was mostly the same (MSVC just had 1 additional
check, which I moved over).

We also only tested `u8string` with MSVC. So i moved those into
`generic` as-is. I kept it separate from the existing std::string tests
since it requires c++20.

The tests are currently failing for libc++ and libstdc++ because MSVC
had a test case which checked that:
```
  std::string overwritten_zero("abc");
  const_cast<char *>(overwritten_zero.data())[3] = 'd';
```
prints as `"abc"`. But libc++ and libstdc++ print it as `"abcd"` (which
seems like the more correct thing to do?)

This combines the libc++ and libstdc++ test cases. The main difference
was that the libstdcpp tests had additional tests for
references/pointers to std::string. I moved those over.

The libstdc++ formatters don't support `std::u16string`/`std::u32string`
yet, so I extracted those into XFAILed test cases.

There were also two test assertions that failed for libstdc++:
1. libstdc++ doesn't obey the capped/uncapped summary options
2. When a summary isn't available for a std::string, libc++ would print
"Summary Unavailable", whereas libstdc++ just prints "((null))". This
may be better suited for the STL-specific subdirectories, but left it
here for now.

I put those in separate XFAILed test-cases.

Split out from https://github.com/llvm/llvm-project/pull/146740