[clang] [libc++][lit] Allow overriding the executor for tests (PR #66545)

[Louis Dionne via cfe-commits]

ldionne wrote:

> > @arichardson
>
> Sorry about that! It looks like the cmake file quoting needs to be updated. Alternatively I could revert the part of this change that deprecates the executor variable.

Let's not un-deprecate the executor variables -- it's really great that we're removing some `LIBCXX_FOO` variables from the CMake build if they are only used for testing.

https://github.com/llvm/llvm-project/pull/66545