[clang-tools-extra] Fix linking ClangdTests when using libclang-cpp

Fix linking ClangdTests to specify the dependency on the private
clangTesting library via target_link_libraries() rather than
clang_target_link_libraries().  The latter uses libclang-cpp when
CLANG_LINK_CLANG_DYLIB is used, and clangTesting is not included
in this library.

This fixes d60d345.

Differential Revision: https://reviews.llvm.org/D146427

clang-tools-extra/clangd/unittests/CMakeLists.txt

@@ -157,7 +157,6 @@ clang_target_link_libraries(ClangdTests
   clangLex
   clangSema
   clangSerialization
-  clangTesting
   clangTooling
   clangToolingCore
   clangToolingInclusions
@@ -172,6 +171,7 @@ target_link_libraries(ClangdTests
 
   clangDaemon
   clangIncludeCleaner
+  clangTesting
   clangTidy
   clangdSupport
   )