[llvm] [lit] Move %clang_dxc substitution from clang/test

Move the `%clang_dxc` substitution from local definition in clang/test
to lit's `llvm/config.py` module where all other driver definitions
are found.  This improves consistency and makes it easier to control
global clang options.

Differential Revision: https://reviews.llvm.org/D134871