
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.
See docs/CMake.html for instructions on how to build LLVM with CMake.