[MS] Treat ignored explicit calling conventions as an explicit __cdecl

[MS] Treat ignored explicit calling conventions as an explicit __cdecl

The CCCR_Ignore action is only used for Microsoft calling conventions,
mainly because MSVC does not warn when a calling convention would be
ignored by the current target. This behavior is actually somewhat
important, since windows.h uses WINAPI (which expands to __stdcall)
widely. This distinction didn't matter much before the introduction of
__vectorcall to x64 and the ability to make that the default calling
convention with /Gv. Now, we can't just ignore __stdcall for x64, we
have to treat it as an explicit __cdecl annotation.

Fixes PR42531

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@365579 91177308-0d34-0410-b5e6-96231b3b80d8