driver: Rework for_each_path using C++HEADtrunkmaster

The old C-style was cumbersome make making one responsible for manually
creating and passing in two parts a closure (separate function and
*_info class for closed-over variables).

With C++ lambdas, we can just:

- derive environment types implicitly
- have fewer stray static functions

Also thanks to templates we can
- make the return type polymorphic, to avoid casting pointee types.

Note that `struct spec_path` was *not* converted because it is used
multiple times. We could still convert to a lambda, but we would want to
put the for_each_path call with that lambda inside a separate function
anyways, to support the multiple callers. Unlike the other two
refactors, it is not clear that this one would make anything shorter.
Instead, I define the `operator()` explicitly. Keeping the explicit
struct gives us some nice "named arguments", versus the wrapper function
alternative, too.

gcc/ChangeLog:

	* gcc.cc (for_each_path): templated, to make passing lambdas
	possible/easy/safe, and to have a polymorphic return type.
	(struct add_to_obstack_info): Deleted, lambda captures replace
	it.
	(add_to_obstack): Moved to lambda in build_search_list.
	(build_search_list): Has above lambda now.
	(struct file_at_path_info):  Deleted, lambda captures replace
	it.
	(file_at_path): Moved to lambda in find_a_file.
	(find_a_file): Has above lambda now.
	(struct spec_path_info): Reamed to just struct spec_path.
	(struct spec_path): New name.
	(spec_path): Rnamed to spec_path::operator()
	(spec_path::operator()): New name
	(do_spec_1): Updated for_each_path call sites.

Signed-off-by: John Ericson <git@JohnEricson.me>
Reviewed-by: Jason Merrill <jason@redhat.com>

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