[docs] Document what "NFC" means in a commit message.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217292 91177308-0d34-0410-b5e6-96231b3b80d8
Fix right shift by 64 bits detected on CXX/lex/lex.literal/lex.ext/p4.cpp
test case on UBSan bootstrap bot.

This fixes the last failure of "check-clang" in UBSan bootstrap bot.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217294 91177308-0d34-0410-b5e6-96231b3b80d8