[MS Compat] Adjust thiscall to cdecl when deducing template arguments

Function types can be extracted from member pointer types.
However, the type is not appropriate without first adjusting the calling
convention.

This fixes PR25661.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@254323 91177308-0d34-0410-b5e6-96231b3b80d8