[LLD] [MinGW] Allow linking to DLLs directly

As the COFF linker is capable of linking directly against a DLL now
(after D104530, as long as it is running in mingw mode), don't error
out here but successfully load libraries specified with "-l" from DLLs
if that's what ld.bfd would have matched.

Differential Revision: https://reviews.llvm.org/D104531