Use LLVM's new error handler API to report back end errors using Diagnostic.

For example,
--
ddunbar@giles:Frontend$ clang -c backend-errors.c
fatal error: error in backend: Global variable 'a' has an invalid section specifier
      'I AM, not, legal': mach-o section specifier uses an unknown section type.
--
compare to:
--
ddunbar@giles:Frontend$ gcc -c backend-errors.c
/var/folders/DQ/DQ8GT3++HESEzT1obWBynE+++TI/-Tmp-//cc45w2pq.s:2:Expected comma after segment-name
/var/folders/DQ/DQ8GT3++HESEzT1obWBynE+++TI/-Tmp-//cc45w2pq.s:2:Rest of line ignored. 1st junk character valued 77 (M).
--

Yay!

I am not tied to my wording choice, we could also go with "uncoverable error"
for the prefix, or just leave it off entirely.

llvm-svn: 78554