[mingw] Don't mangle thiscall like fastcall etc

GCC does not mangle it when it is not explicit in the source.  The
mangler as currently written cannot differentiate between explicit and
implicit calling conventions, so we can't match GCC. Explicit thiscall
conventions are rare, so mangle as if the convention was implicit to be
as ABI compatible as possible.

Also fixes some tests using %itanium_abi_triple in some configurations
as a side effect.

Fixes PR40107.

llvm-svn: 349872