backend/xcode: replace use of CustomTarget method with open code

This function isn't supposed to be used for build targets, so just open
code this instead. This also gets xcode fully type safe.

mesonbuild/backend/xcodebackend.py

@@ -1747,7 +1747,8 @@ def generate_single_build_target(
                         # It is unclear what is the cwd when xcode runs. -I. does not seem to
                         # add the root build dir to the search path. So add an absolute path instead.
                         # This may break reproducible builds, in which case patches are welcome.
-                        cti_args = self.get_custom_target_dir_include_args(target, compiler, absolute_path=True)
+                        cti_args.append('-I{}'.format(os.path.join(self.environment.source_dir, target.subdir)))
+                        cti_args.append('-I{}'.format(os.path.join(self.environment.build_dir, target.subdir)))
                     # Xcode cannot handle separate compilation flags for C and ObjectiveC. They are both
                     # put in OTHER_CFLAGS. Same with C++ and ObjectiveC++.
                     if lang == 'objc':

run_mypy.py

@@ -37,6 +37,7 @@
     'mesonbuild/backend/vs2017backend.py',
     'mesonbuild/backend/vs2019backend.py',
     'mesonbuild/backend/vs2022backend.py',
+    'mesonbuild/backend/xcodebackend.py',
     # 'mesonbuild/coredata.py',
     'mesonbuild/depfile.py',
     'mesonbuild/envconfig.py',