Use llvm-symbolizer to symbolize LLVM/Clang crash dumps.

This change modifies fatal signal handler used in LLVM tools.
Now it attempts to find llvm-symbolizer binary and communicates
with it in order to turn instruction addresses into
function/file/line info entries. This should significantly improve
stack traces readability in Debug builds.

This feature only works on selected platforms (including Darwin
and Linux). If the symbolization fails for some reason, signal
handler will fallback to the original behavior.

Reviewed in http://reviews.llvm.org/D5610

llvm-svn: 219354