This allows passing parameters to the test suites without using
LLVM_LIT_ARGS. The problem is that we sometimes want to set some
Lit arguments on the CMake command line, but the Lit parameters in
a CMake cache file. If the only knob to do that is LLVM_LIT_ARGS,
the command-line entry overrides the cache one, and the parameters
set by the cache are ignored.

This fixes a current issue with the build bots that they completely
ignore the 'std' param set by Lit, because other Lit arguments are
provided via LLVM_LIT_ARGS on the CMake command-line.