[libclang] Modify clang_findReferencesInFile and clang_findIncludesInFile to return a value.

Possible values are:
 1  : if a parameter was invalid
 -1 : if the callback returned CXVisit_Break,
 otherwise returns 0.
[PCH] When complaining that a header from the PCH was modified, also mention
the filename of the PCH file.