Write more tests for `target_build_settings` generator

In order to get incremental generation mode into a proper build for additional widespread testing, I deferred some tests:

• Write ParseTransitiveSwiftDebugSettings tests
• Write ProcessArgs tests
• Write ProcessCArgs tests
• Write ProcessCcArg tests
• Write ProcessCcArgs tests
• Write ProcessCxxArgs tests
• Write ProcessSwiftArg tests
• Write ProcessSwiftArgs tests
• Write ProcessSwiftClangArg tests
• Write ProcessSwiftFrontendArg tests
• Write WriteBuildSettings tests

Locate the target_build_settings generator and its existing tests, then trace the listed entry points such as ParseTransitiveSwiftDebugSettings, ProcessArgs, and WriteBuildSettings. Add coverage for each deferred function and run the generator's test suite; done means all listed test items are covered.