[clang-shlib] Don't link with static clang libraries

Summary:
If we are building static libraries we don't need to link them into
clang-shlib, since clang-shlib already has all the individual object
files linked in.

Reviewers: smeenai

Reviewed By: smeenai

Subscribers: mgorny, cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82694