[COFF] Avoid forward declaring StringSet, fix build

This should fix the build after SVN r316178, which worked fine
on GCC 5.4, but failed on clang with errors like these:

MinGW.h:26:3: error: too few template arguments for class template 'StringSet'
  StringSet<> ExcludeSymbols;
  ^
lld/Common/LLVM.h:28:30: note: template is declared here
  template<typename T> class StringSet;

Don't forward declare and add the using directive in the main
lld/Common/LLVM.h header, but just qualify the class name
in MinGW.h instead.

llvm-svn: 316180