[MinGW] Ignore -fvisibility/-fvisibility-inlines-hidden for dllexport

Similar to 123ce97f for dllimport: dllexport
expresses a non-hidden visibility intention. We can consider it explicit and
therefore it should override the global visibility setting (see AST/Decl.cpp
"NamedDecl Implementation").

Adding the special case to CodeGenModule::setGlobalVisibility is somewhat weird,
but allows we to add the code in one place instead of many in AST/Decl.cpp.

Differential Revision: https://reviews.llvm.org/D133180