Make MSan unittest -Werror=sign-compare clean

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204592 91177308-0d34-0410-b5e6-96231b3b80d8
[CMake] Respect CMAKE_CXX_FLAGS in custom clang_compile commands

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204593 91177308-0d34-0410-b5e6-96231b3b80d8