One way to currently test the reproducers is to setup
"FORCE_CLANG_DIAGNOSTICS_CRASH=1" before invoking clang. This simulates
a crash and produces the same contents needed by the reproducers.  The
reproducers are specially useful when triaging Modules issues, not only
on crashes, but also for reproducing misleading warnings, errors, etc.

Add a '-gen-reproducer' driver option to clang (or any similar name) and
give users a flag option.

Note that clang already has a -fno-crash-diagnostics, which disables the
crash reproducers. I've decided not to propose "-fcrash-diagnostics"
since it doesn't convey the ideia of reproduction despite a crash.

rdar://problem/24114619

Differential Revision: https://reviews.llvm.org/D27604

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300109 91177308-0d34-0410-b5e6-96231b3b80d8