Fix standalone build interaction with compiler extension

As suggested in https://github.com/llvm/llvm-project/issues/120, don't try to
generate the extension file from clang, only do the linking step.

Fixes the regression introduced in D74464 when running cmake inside the clang
directory.

Differential Revision: https://reviews.llvm.org/D74602

(cherry picked from commit 87dac7da)