[CMake] Cleanup LLVM_OPTIMIZED_TABLEGEN

[CMake] Cleanup LLVM_OPTIMIZED_TABLEGEN

This cleanup removes the need for the native support library to have its own target. That target was only needed because makefile builds were tripping over each other if two tablegen targets were building at the same time. This causes problems because the parallel make invocations through CMake can't communicate with each other. This is fixed by invoking make directly instead of through CMake which is how we handle this in External Project invocations.

The other part of the cleanup is to mark the custom commands as USES_TERMINAL. This is a bit of a hack, but we need to ensure that Ninja generators don't invoke multiple tablegen targets in the same build dir in parallel, because that too would be bad.

Marking as USES_TERMINAL does have some downside for Ninja because it results in decreased parallelism, but correct builds are worth the minor loss and LLVM_OPTIMZIED_TABLEGEN is such a huge win, it is worth it.

llvm-svn: 280748