Make crashlog.py work when a .dSYM is present, but a binary is missing

Often users have a crash log an d a .dSYM bundle, but not the original
application binary. It turns out that for crash symbolication, we can
safely fall back to using the binary inside the .dSYM bundle.

Differential Revision: https://reviews.llvm.org/D55607

llvm-svn: 349366