Added -fcrash-diagnostics-dir flag

Summary:
New flag causes crash reports to be written in the specified directory
rather than the temp directory.

Patch by Chijioke Kamanu.

Reviewers: hans, inglorion, rnk

Reviewed By: hans

Subscribers: zturner, hiraditya, llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D48601

llvm-svn: 336604