MFC r267335:

In some scenarios, when generating llvm/clang .inc.h files, multiple
source files could be passed to tblgen or clang-tblgen, leading to a
"Too many positional arguments specified" error message.  Fix this by
replacing the too-generic ${.ALLSRC} sources with explicit paths.

Reported by:	rysto32@gmail.com, rodrigc
Approved by:	re (gjb)