[PATCH] Optional support for dynamic Asan runtime

[Yury Gribov]

  First try on Konstantin's suggestions in ML (https://groups.google.com/forum/#!topic/address-sanitizer/XJXOrSvN8vg).

  Stuff that hasn't been addressed:
  * platforms other than Linux
  * support for other dynamic runtimes (UBSan, MSan, etc.)
  * detection of static/dynamic rt conflict
  * detection of dlopen

http://llvm-reviews.chandlerc.com/D3042