[compiler-rt] Fix passing debug information to unittest.

Summary:
On windows, the debug information was not present in the unittest executables,
which make them hard to debug.

The Sanitizer Unittests are compiled with a local clang build.
The link pass is also done by calling clang.

This pass is adding the appropriate flags to bring the right debug information
to these executables.

Reviewers: rnk

Subscribers: kubabrecka, llvm-commits, wang0109, chrisha

Differential Revision: http://reviews.llvm.org/D21838

llvm-svn: 274153