[libcxx] Add special warning flag detection logic to compiler.py

Summary: Detecting `-Wno-<warning>` flags can be tricky with GCC (See https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html). This patch adds a special `addWarningFlagIfSupported(<flag>)` method to the test compiler object that can be used to add warning flags. The goal of this patch is to help get the test suite running with more warnings.

Reviewers: danalbert, jroelofs

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D11333

llvm-svn: 246069