Silence the driver warnings, if we see "-w" on the Driver.

Summary:
We can enable warnings after that -w, so the patch might not be 100%
correct.

The problem that triggered this is: we have some amount of tests that
expect 0 warnings (including unit tests for -w), but -w ends up not fully
silencing everything.

Reviewers: echristo, chandlerc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D11322

llvm-svn: 242606