MFC r266053:

Use the new -d option that was added to tblgen between llvm/clang 3.3
and 3.4 to generate dependency files for the '.inc.h' files generated
from .td files, and .sinclude those dependency files in clang.build.mk.

This will make future incremental builds of lib/clang and usr.bin/clang
work correctly, whenever any of the .td files get modified.

Note that this will not fix any problems with incremental builds from
*before* this revision, since there will not yet be any generated
dependency files.  A quick workaround is to run the following:

  find /usr/obj -type f -name '*.inc.h' | xargs rm

and then a regular incremental buildworld (e.g. with -DNO_CLEAN).