Refine report_fatal_error guidance after post-commit review

Use text suggested by Justin Bogner in post-commit review of r311146 
<http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20170814/479898.html>, 
which makes it clear that report_fatal_error shouldn't be used when there is a 
practicable alternative. Also make this clearer in CodingStandards.

llvm-svn: 311147