Fix a copypasto bug. Also rename the parameter in question to not shadow the

'file' builtin, and fix up a docstring a little.

Hat tip to Sebastian Kreft Carreno at Google for noticing the bug.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@169887 91177308-0d34-0410-b5e6-96231b3b80d8