[gn build] Make sync_source_lists_from_cmake.py check that all LLVM unittests are present

[thakis]

Summary

After the PowerPC and WebAssembly targets are added, this check passes.

[phosek]

LGTM

llvm/utils/gn/build/sync_source_lists_from_cmake.py

for gn_file in gn_files:
             print('remove:\n' + '\n'.join(remove))
         print()
 
 
 def sync_unittests():
     # Matches e.g. |add_llvm_unittest_with_input_files|.
     unittest_re = re.compile(r'^add_\S+_unittest', re.MULTILINE)
 
-    # FIXME: Add 'llvm' here once it's complete.
-    checked = [ 'clang', 'lld' ]
+    checked = [ 'clang', 'lld', 'llvm' ]
     for c in checked:
         for root, _, _ in os.walk(os.path.join(c, 'unittests')):
             cmake_file = os.path.join(root, 'CMakeLists.txt')
             if not os.path.exists(cmake_file):
                 continue
             if not unittest_re.search(open(cmake_file).read()):
                 continue  # Skip CMake files that just add subdirectories.
             gn_file = os.path.join('llvm/utils/gn/secondary', root, 'BUILD.gn')