[PATCH] [msan] Unpoison siginfo_t and ucontext_t in signal handlers.

[Michael Gottesman]

  This is causing buildbot failures.

  Phased Builder: http://lab.llvm.org:8013/builders/phase1%20-%20sanity/builds/4461
  Subbuilder:         http://lab.llvm.org:8013/builders/clang-x86_64-darwin11-nobootstrap-RA/builds/3332

http://llvm-reviews.chandlerc.com/D611