Revert "[Clang] [AST] Fix placeholder return type name mangling for MSVC 1920+ / VS2019+ (#102848)"

It cause builds to start failing with

  Invalid type expected
  UNREACHABLE executed at clang/lib/AST/MicrosoftMangle.cpp:2551!

see comments on the PR.

> Partial fix for https://github.com/llvm/llvm-project/issues/92204.
> This PR just fixes VS2019+ since that is the suite of compilers that I
> require link compatibility with at the moment.
> I still intend to fix VS2017 and to update llvm-undname in future PRs.
> Once those are also finished and merged I'll close out
> https://github.com/llvm/llvm-project/issues/92204.
> I am hoping to get the llvm-undname PR up in a couple of weeks to be
> able to demangle the VS2019+ name mangling.
>
> MSVC 1920+ mangles placeholder return types for non-templated functions
> with "@".
> For example `auto foo() { return 0; }` is mangled as `?foo@@YA@XZ`.
>
> MSVC 1920+ mangles placeholder return types for templated functions as
> the qualifiers of the AutoType followed by "_P" for `auto` and "_T" for
> `decltype(auto)`.
> For example `template<class T> auto foo() { return 0; }` is mangled as
> `??$foo@H@@YA?A_PXZ` when `foo` is instantiated as follows `foo<int>()`.
>
> Lambdas with placeholder return types are still mangled with clang's
> custom mangling since MSVC lambda mangling hasn't been deciphered yet.
> Similarly any pointers in the return type with an address space are
> mangled with clang's custom mangling since that is a clang extension.
>
> We cannot augment `mangleType` to support this mangling scheme as the
> mangling schemes for variables and functions differ.
> auto variables are encoded with the fully deduced type where auto return
> types are not.
> The following two functions with a static variable are mangled the same
> ```
> template<class T>
> int test()
> {
>     static int i = 0; // "?i@?1???$test@H@@YAHXZ@4HA"
>     return i;
> }
>
> template<class T>
> int test()
> {
>     static auto i = 0; // "?i@?1???$test@H@@YAHXZ@4HA"
>     return i;
> }
> ```
> Inside `mangleType` once we get to mangling the `AutoType` we have no
> context if we are from a variable encoding or some other encoding.
> Therefore it was easier to handle any special casing for `AutoType`
> return types with a separate function instead of using the `mangleType`
> infrastructure.

This reverts commit e0d173d44161bf9b68243845666d58999e74f759
and the wollow-up fa343be414f9364911b947f109f3df5539e23068.

0 files changed, 0 insertions, 0 deletions