[Sema] Move pointer to int cast warnings under -Wmicrosoft-cast

[aeubanks]

Summary

Microsoft extensions should be under a -Wmicrosoft-foo group.
Added in https://reviews.llvm.org/D75708.

There shouldn't be a reason to separate out void pointers since this is C++.

[rnk]

I agree with the reasoning, but this is likely to fire all over existing C++ code. In fact, the one we just added finds issues in Chrome
https://ci.chromium.org/p/chrome/builders/ci/ToTWin/5879
../../base/debug/close_handle_hook_win.cc(155,16): error: cast to smaller integer type 'unsigned long' from 'void *'

The idea was that -Wpointer-int-cast was a new warning group, so it was fine to add new findings to it, but -Wmicrosoft-cast already exists. In this case, I would recommend adding some new -W flag.

As for naming, -Wmicrosoft-pointer-to-int? It can still be a subgroup of -Wmicrosoft-cast.

[aeubanks]

Specifically regarding
../../base/debug/close_handle_hook_win.cc(155,16): error: cast to smaller integer type 'unsigned long' from 'void *',
I sent out a change to fix that: https://chromium-review.googlesource.com/c/chromium/src/+/2091215, and tried compiling with ToT clang with the suppression removed, then there were no more errors.

Anyway, we should probably be consistent between this and https://reviews.llvm.org/D75643. I'm fine with keeping what we have, but then https://reviews.llvm.org/D75643 should probably go in.

clang/lib/Sema/SemaCast.cpp

if (DestType->isIntegralType(Self.Context)) {
     // C++ 5.2.10p4: A pointer can be explicitly converted to any integral
     //   type large enough to hold it; except in Microsoft mode, where the
     //   integral type size doesn't matter (except we don't allow bool).
     if ((Self.Context.getTypeSize(SrcType) >
          Self.Context.getTypeSize(DestType))) {
       bool MicrosoftException =
           Self.getLangOpts().MicrosoftExt && !DestType->isBooleanType();
       if (MicrosoftException) {
-        unsigned Diag = SrcType->isVoidPointerType()
-                            ? diag::warn_void_pointer_to_int_cast
-                            : diag::warn_pointer_to_int_cast;
-        Self.Diag(OpRange.getBegin(), Diag) << SrcType << DestType << OpRange;
+        Self.Diag(OpRange.getBegin(), diag::ext_ms_pointer_to_int_cast)
+            << SrcType << DestType << OpRange;
       } else {
         msg = diag::err_bad_reinterpret_cast_small_int;
         return TC_Failed;
       }
     }
     Kind = CK_PointerToIntegral;
     return TC_Success;
   }