This change lets us bootstrap LLVM/Clang under ASan and MSan. It contains
fixes for 2 issues:

- X86JIT reads return address from stack, which MSan does not know is
  initialized.
- bugpoint tests run binaries with RLIMIT_AS. This does not work with certain
  Sanitizers.

We are no longer including config.h in Compiler.h with this change.

llvm-svn: 174306