[libcxx] Add support for LLVM_USE_SANITIZER to libcxx when being built standalone and in-tree

Summary:
This patch adds support for LLVM_USE_SANITIZER when being built in-tree and standalone.

This patch does the following things:
1. define the LLVM_USE_SANITIZER option to "" when being built standalone. This also helps show we support it.
2. Translate LLVM_USE_SANITIZER when standalone in a very similar way done in llvm/cmake/HandleLLVMOptions.cmake.
3. Add config.llvm_use_sanitizer to lit.site.cfg.in
4. Add code to translate config.llvm_use_sanitizer's value into the needed compile flags in lit.cfg.

Currently lit.cfg assumes that that the compiler supports '-fno-omit-frame-pointer' while CMakeLists.txt actually checks to see if its supported. We could pass this information to lit but I'm not sure its needed.

Reviewers: mclow.lists, danalbert

Reviewed By: danalbert

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4949