Refactor string checking. No functionality change.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181824 91177308-0d34-0410-b5e6-96231b3b80d8
Add 'CHECK-DAG' support

Refer to 'FileCheck.rst'f for details of 'CHECK-DAG'.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181827 91177308-0d34-0410-b5e6-96231b3b80d8