[Clang] Add support for `/Zc:enumTypes` compiler flag in MSVC mode

[Saalvage]

MSVC used to have a bug where enums always had the implicit underlying type of int , independently of the enum's items. This bug is accomodated for within clang's MSVC compatibility mode, while the fix for it (the /Zc:enumTypes flag) is not.

The issue with this is that this results in information being lost for the enum's items (as they are trimmed to fit the underlying type), so fixing this on the application side would likely require reparsing the items.

I believe this might potentially be a good fit for an option in LangOptions.def?

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Stefan (Saalvage)

MSVC used to have a [bug](https://developercommunity.visualstudio.com/t/underlying-type-of-an-unscoped-enum/524018) where enums always had the implicit underlying type of `int` , independently of the enum's items. This bug is [accomodated for](https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaDecl.cpp#L17268-L17275) within clang's MSVC compatibility mode, while the fix for it (the `/Zc:enumTypes` flag) is not.

The issue with this is that this results in information being lost for the enum's items (as they are trimmed to fit the underlying type), so fixing this on the application side would likely require reparsing the items.

I believe this might potentially be a good fit for an option in LangOptions.def?

[frederick-vs-ja]

Note that MSVC still thinks the underlying type is int when the enum is incomplete.

It's questionable whether Clang-cl mode should still be bug-compatible with MSVC on this, see also #169472.

Example (Godbolt link):

template<class, class>
constexpr bool is_same_v = false;
template<class T>
constexpr bool is_same_v<T, T> = true;

enum E2 {
  ix = is_same_v<__underlying_type(E2), int>,
  m2 = 0x1'0000'0000ULL,
  iy = is_same_v<__underlying_type(E2), int>,
};
static_assert(ix); // accepted by MSVC, but weird
static_assert(iy); // accepted by MSVC, but weird
static_assert(is_same_v<__underlying_type(E2), long long>);