[clang-tidy] Change the namespace for llvm checkers from 'llvm' to 'llvm_check'

Summary:
Change the namespace for llvm checkers from 'llvm' to
'llvm_check', and modify add_new_check.py and rename_check.py to
support the new namespace. Checker, file, and directory names remain
unchanged.

Used new version of rename_check.py to make the change in existing
llvm checkers, but had to fix LLVMTidyModule.cpp and
LLVMModuleTest.cpp by hand.

The changes made by rename_check.py are idempotent, so if accidentally
run multiple times, it won't do anything.

Reviewed By: aaron.ballman

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D60629

llvm-svn: 360450