[Driver] MinGW: Remove custom linker detection

In rL289668 the ability to specify the default linker at compile time
was added but because the MinGW driver used custom detection we could
not take advantage of this new CMAKE flag CLANG_DEFAULT_LINKER.

This is a re-apply of rL313082 which was reverted in rL313088
due to failing buildbot tests.

Differential Revision: https://reviews.llvm.org/D37727

llvm-svn: 313102