Use buddy as fallback for headers without explicitly pinned TU.
Review Request #122160 - Created Jan. 20, 2015 and submitted - Latest diff uploaded
In such cases, it's worthwile to use the source buddy as best-guess as it allows us to get rid of the parse run for the "header TU" which gets thrown away later on anyways. Performance wise, this patch improves the index run on a 48 file project using STL: It goes from ~17s down to ~14s, just b/c we safe a few needless clang runs on "header TUs". REVIEW: 122160