[msan] Lit tests for MemorySanitizer.

Build system setup for MSan lit tests (build with freshly-built clang, run,
check output) - a nearly exact copy from ASan.
First 2 lit tests for MSan.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@174876 91177308-0d34-0410-b5e6-96231b3b80d8