[FileCheck, NFC] Split defines.txt in two

Summary:
Split defines.txt into diagnostics test and functionality test. Also add
comments, remove the semicolon prefix and group RUN lines with their
CHECK directives.

Reviewers: jhenderson, probinson, arichardson

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61679

llvm-svn: 360289