[MinGW] Skip adding default win32 api libraries if -lwindowsapp is specified

In this setup, skip adding all the default windows import libraries,
if linking to windowsapp (which replaces them, when targeting the
windows store/UWP api subset).

With GCC, the same is achieved by using a custom spec file, but
since clang doesn't use spec files, we have to allow other means of
overriding what default libraries to use (without going all the
way to using -nostdlib, which would exclude everything). The same
approach, in detecting certain user specified libraries and omitting
others from the defaults, was already used in SVN r314138.

Differential Revision: https://reviews.llvm.org/D49059

llvm-svn: 336655

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