[CMake] Unconditionally add .h and .td files to target sources

Previously adding header and table gen files was conditional on using an IDE. Since these files have the `HEADER_FILE_ONLY` attribute applied they are ignored as sources by all non-IDE generators, so there is really no reason not to include them.

Additionally having the CMake always include these files allows the CMake-server to include them in the sources list for targets, which is valuable to anyone using CMake-server integrated tools.

llvm-svn: 344218