Remove unneccessary string copies from method invocations.

Summary:
Change string parameter 'File' to be passed by const-reference to
reduce copies.

Patch by Mitch Phillips

Reviewers: vlad.tsyrklevich

Reviewed By: vlad.tsyrklevich

Subscribers: Eugene.Zelenko, llvm-commits

Differential Revision: https://reviews.llvm.org/D37652

llvm-svn: 312994