[COFF] Fix exporting of functions starting with underscores, etc

This fixes exporting functions in the following cases:
- functions starting with an underscore in def files
- functions starting with an underscore, via dllexport attributes, for mingw
- fastcall and vectorcall functions when declared undecorated in def files
- vectorcall functions when declared decorated in def files
- stdcall functions when declared decorated in def files for mingw

This still exports the stdcall functions with the wrong name
in the normal msvc/link.exe mode, if declared with decoration in
the def file though (this is not a regression though). Exporting
functions via def files including decoration is not something I
believe is routinely done though, but is tested to try to match
link.exe's behaviour as far as easily possible.

Differential Revision: https://reviews.llvm.org/D39170

llvm-svn: 316317