Fix a missing comma between two string literals.

Summary:
The two literals are currently appended.

I'm not sure what was broken by this. Please double check carefully.

Silly bug found by an on-going checker for clang-tidy.

Reviewers: alexfh, arsenm, rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18454

llvm-svn: 264315