Remove llvm:: from uses of ArrayRef.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211987 91177308-0d34-0410-b5e6-96231b3b80d8
Import MutableArrayRef into clang namespace.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211988 91177308-0d34-0410-b5e6-96231b3b80d8