Generate a unique filename for a given header

This patch is in preparation for writing the header replacement to disk.
Added getUniqueHeaderName() that generates a unique header filename in
the same directory as the header file.

Differential Revision: http://llvm-reviews.chandlerc.com/D1104


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@186007 91177308-0d34-0410-b5e6-96231b3b80d8