Use the right calling convention when mangling names in the Microsoft C++

Use the right calling convention when mangling names in the Microsoft C++
mangler. Now member functions and pointers thereof have their calling
convention mangled as __thiscall if they have the default CC (even though,
they technically still have the __cdecl CC).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@118598 91177308-0d34-0410-b5e6-96231b3b80d8