cpplib: Move more switches to the front end

[Neil Booth]

Zack Weinberg wrote:-

> Neil Booth <neil@daikokuya.co.uk> writes:
> 
> > This moves all switches related to include path handling to the front end.
> > It's a minor re-sync of a patch I sent last December.  It also cleans up
> > the logic of building the include chain, and makes it more efficient,
> > though it is still quadratic.
> 
> This looks fine except for one thing: I don't see a need to move
> _cpp_simplify_pathname.  In fact, I think it's a bad idea -- now all
> clients of cpplib must provide this fairly hairy piece of code which
> is going to be functionally identical everywhere it's implemented.

You might be right; let me think about it a bit more.

Neil.