[clang] [unittest/AST] Fix linking to LLVMTestingSupport

Link to LLVMTestingSupport via target_link_libraries() instead of
clang_target_link_libraries().  The latter is ineffective if tests
are linked to libclang.so.  This solution is consistent with what other
tests do.

clang/unittests/AST/CMakeLists.txt

@@ -43,5 +43,9 @@ clang_target_link_libraries(ASTTests
   clangFrontend
   clangSerialization
   clangTooling
-  LLVMTestingSupport
   )
+
+target_link_libraries(ASTTests
+  PRIVATE
+  LLVMTestingSupport
+)