Use ConstStringRef facility for getCustomDiagID() safety

This is one of various functions in clang that don't handle arbitrary strings
well and can benefit from compile-time safety checks.

Also fixes an off-by-one error that caused one additional null byte to get
added to the end of custom diagnostic descriptions. ConstStringRef handles
tricky details like that for us now.

Requires supporting changes in LLVM r200187.

llvm-svn: 200190