[Support] Make report_fatal_error respect its GenCrashDiag argument so it doesn't...

[Support] Make report_fatal_error respect its GenCrashDiag argument so it doesn't generate a backtrace

There are a few places where we use report_fatal_error when the input is broken.
Currently, this function always crashes LLVM with an abort signal, which
then triggers the backtrace printing code.
I think this is excessive, as wrong input shouldn't give a link to
LLVM's github issue URL and tell users to file a bug report.
We shouldn't print a stack trace either.

This patch changes report_fatal_error so it uses exit() rather than
abort() when its argument GenCrashDiag=false.

Reviewed by: nikic, MaskRay, RKSimon

Differential Revision: https://reviews.llvm.org/D126550