[PATCH] Add CMake option LLVM_USE_SANITIZER to allow bootstrap of LLVM/Clang under ASan/MSan.

[Alexey Samsonov]

  I've commited non-sanitizer part of this patch as r176973.
  Re another comments:
  * I've enclosed sanitizer flag setup in "if (LLVM_ON_UNIX)"
  * Potentially we can use UBSan together with other sanitizers, but for now I don't want to create too many possible "sanitizer" configurations, or add too much flexibility reflecting combinations of -fsanitize= values.

http://llvm-reviews.chandlerc.com/D459