[clang] SFINAE context refactor (#164703)
This teases the SFINAE handling bits out of the CodeSynthesisContext,
and moves that functionality into SFINAETrap and a new class.
There is also a small performance benefit here:
<img width="1460" height="20" alt="image"
src="https://github.com/user-attachments/assets/aeb446e3-04c3-418e-83de-c80904c83574"
/>
