[clang-tools-extra] Fix overzealous linking of dylib to clangTidy

Fix accidentally making clangTidy library link to dylib.  This causes
libclang.so to also link to dylib which results in duplicate symbols
from shared and static libraries, and effectively to registering
command-line options twice.

Thanks to Sylvestre Ledru for noticing this and tracking it down
to r373786.  Fixes PR#43589.

Differential Revision: https://reviews.llvm.org/D68927

llvm-svn: 374885