Give guidance on report_fatal_error in CodingStandards.rst and ProgrammersManual.rst

The current ProgrammersManual.rst document has a lot of well-written 
documentation on error handling thanks to @lhames. It suggests errors can be 
split cleanly into "programmatic" and "recoverable" errors. However, the 
reality in current LLVM seems to be there are a number of cases where a 
non-programmatic error is not easily recoverable. Therefore, add a note to 
indicate the existence of report_fatal_error for these cases. I've also added 
a reminder to CodingStandards.rst in the section on assertions, to indicate 
that llvm_unreachable and assertions should not be relied upon to report 
errors triggered by user input.

The ProgrammersManual is also silent on the use of LLVMContext::diagnose, 
which is used in BPF+WebAssembly+AMDGPU to report some errors during 
instruction selection. I don't address that in this patch, as it's not quite 
clear how to fit in to the current error handling story

Differential Revision: https://reviews.llvm.org/D36826

llvm-svn: 311146