[llvm-rc] Respect the executable specified in the --preprocessor command

The arguments passed in this option were passed onto the child
process, but we still blindly used the clang binary that we had
found to sys::ExecuteAndWait as the intended executable to run.

If the user hasn't specified any custom --preprocessor command,
Args[0] is equal to the variable Clang.

This doesn't affect any tests, since the tests only print the
arguments it would try to execute (but not the first parameter to
sys::ExecuteAndWait), but there's no testes for executing it
(and validating that it did execute the right thing).

Differential Revision: https://reviews.llvm.org/D146793