clang-scan-deps: Pass ${TDFILE} to tblgen, not ${.ALLSRC}

${.ALLSRC} can include the dependency OptParser.td, which causes
llvm-tblgen to fail since it only accepts a single input argument.

Use ${TDFILE} directly instead, which matches the other invocations
of tblgen in the LLVM Makefiles.

Fixes: d3c06bed2c16 ("clang: install clang-scan-deps")
MFC after: 3 days
Reviewed by: dim
Differential Revision: https://reviews.freebsd.org/D51569

(cherry picked from commit ad023bc26e106e28e1b4845f991b52b533bd0802)