Add clang support of RTM from TSX

- New options '-mrtm'/'-mno-rtm' are added to enable/disable RTM feature
- Builtin macro '__RTM__' is defined if RTM feature is enabled
- RTM intrinsic header is added and introduces 3 new intrinsics, namely
  '_xbegin', '_xend', and '_xabort'.
- 3 new builtins are added to keep compatible with gcc, namely
  '__builtin_ia32_xbegin', '__builtin_ia32_xend', and '__builtin_ia32_xabort'.
- Test cases for pre-defined macro and new intrinsic codegen are added.

llvm-svn: 167665