TableGen.cmake: sometimes the .td file is not in the current directory

(clang/include/clang/Basic/StmtNodes.td, for instance, is tablegenned
from clang/include/clang/AST/CMakeLists.txt) so it is not contained on
the list of all .td files on the current source directory which is
used as the DEPENDS of the custom command. We must add the .td file to
the DEPENDS list of the custom command. Otherwise some .inc files are
not regenerated when the corresponding .td file changes.

llvm-svn: 122768