[CMake] Allow devs to disable implicit incremental testing

[davezarzycki]

Lit's incremental mode is incompatible with read-only source filesystems which some CI setups use and there is no way to disable incremental mode in Swift without enabling --shuffle via LLVM_LIT_ARGS.

[davezarzycki]

@swift-ci please smoke test

[compnerd]

What do you think of changing this to something like:

set(SWIFT_LIT_ARGS "--incremental" CACHE STRING
  "arguments to pass to lit")

This will allow you to override the parameter without an option while allowing greater control over the arguments.

If you think that this is too much freedom, the option makes sense.

[davezarzycki]

That seems better than what I proposed.

@swift-ci please smoke test