[ms-coff] Add a test for proper handling of full Windows path names in the .drectve section

Summary: This test ensures that we can correctly specify a full Windows path to the clang ASAN runtime libraries.  This is in preparation to fix PR20246.

Reviewers: rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4427

llvm-svn: 212580