[clang] Fix linking to LLVMTestingAnnotations in standalone build

[mgorny]

Summary

The LLVMTestingAnnotations library that is now used by unittests
is not installed as part of LLVM. In order to make it possible to build
unittests when performing the standalone build of clang, build
the library from LLVM sources locally. This mirrors the existing logic
for LLVMTestingSupport.

[rupprecht]

Thanks!

[mgorny]

Thanks for the superfast review! I'll push it later today.

clang/unittests/CMakeLists.txt

 add_custom_target(ClangUnitTests)
 set_target_properties(ClangUnitTests PROPERTIES FOLDER "Clang tests")
 
 if(CLANG_BUILT_STANDALONE)
-  # LLVMTestingSupport library is needed for some of the unittests.
+  # LLVMTesting* libraries are needed for some of the unittests.
+  if (EXISTS ${LLVM_MAIN_SRC_DIR}/lib/Testing/Annotations
+      AND NOT TARGET LLVMTestingAnnotations)
+    add_subdirectory(${LLVM_MAIN_SRC_DIR}/lib/Testing/Annotations
+      lib/Testing/Annotations)
+  endif()
   if (EXISTS ${LLVM_MAIN_SRC_DIR}/lib/Testing/Support
       AND NOT TARGET LLVMTestingSupport)
     add_subdirectory(${LLVM_MAIN_SRC_DIR}/lib/Testing/Support
       lib/Testing/Support)
   endif()
 endif()
 
 # add_clang_unittest(test_dirname file1.cpp file2.cpp)