[CMake] Fix for PR 23328: LLVM_OPTIMIZED_TABLEGEN broken

In CMake dependencies can be filenames or targets, and targets can't be filenames. The Ninja generator handles filename dependencies because it generates targets for every output file from a command. For example:

add_custom_command(OUTPUT foo.txt COMMAND touch foo.txt)

With the Ninja generator this generates a target foo.txt, but with the Makefile generator it doesn't. This is probably because Ninja explicitly requires these hard dependency ties, and Make just behaves oddly in general.

To fix this we need to make the tablegen actions depend on a target rather than a filename.

llvm-svn: 235732