Simplify MicrosoftCXXNameMangler::mangleFunctionType

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191950 91177308-0d34-0410-b5e6-96231b3b80d8
Revert r191947. The problem is not MIPS-specific and requires more
general solution.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191951 91177308-0d34-0410-b5e6-96231b3b80d8