[CMake][LIT] Add option to run lit testsuites in parallel (#82899)

Currently `add_lit_target` sets the `USES_TERMINAL` CMake option. When
using Ninja, this forces all lit testsuite targets into the
single-threaded `console` pool.

This PR adds a new option `LLVM_PARALLEL_LIT` which drops the
`USES_TERMINAL` flag, allowing Ninja to run them in parallel.

The default setting (`LLVM_PARALLEL_LIT=OFF`) retains the existing
behavior of serial testsuite execution.