[FileCheck] Fix code style of method comments

Summary:
Fix various issues in code style of method comments:
1) Move all heading comments to all non-static methods near their
declaration in the FileCheck.h header file.
2) Harmonize the action verb in doxygen comments for methods to always
be in third person
3) Use \returns instead of free text "return" and "returns".
4) Document a couple more parameters while at it.

Reviewers: jhenderson, probinson, arichardson

Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61445

llvm-svn: 360288