[FileCheck] Remove llvm:: prefix

Summary:
Remove all llvm:: prefixes in FileCheck library header and
implementation except for calls to make_unique and make_shared since
both files already use the llvm namespace.

Reviewers: jhenderson, jdenny, probinson, arichardson

Subscribers: hiraditya, arichardson, probinson, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62323

llvm-svn: 361515