[PATCH] Add new lit argument: --exec-feature=FEATURE

[Alexey Samsonov]

  Hi Daniel,

  Thanks for pointing out at llvm_use_intel_jitevents! I think we can benefit from adding a build configuration options that would tell
  the build system to build LLVM/Clang under ASan/MSan/whatever. Then we can setup the correct compile flags, alter llvm-lit options
  etc.

  Still, I think this is somewhat orthogonal to this patch: we want to mark certain tests as "xfailing" under ASan, not "unsupported".
  It's also good if we have per-test granularity instead of per-directory offered by lit.site.cfg (which is far less discoverable).

  Probably, there is another way to achieve the goal - specify "asan" in --param flag, but this would also require untrivial
  changes to lit to make it respect --param values in XFAIL: lines.

  P.S. One more random crazy idea: we can later pull --vg and --vg-leak under --exec-feature flag :)

http://llvm-reviews.chandlerc.com/D413