Refine report_fatal_error guidance after post-commit review

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311147 91177308-0d34-0410-b5e6-96231b3b80d8