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
Martin Storsjo