gccrs: Collect error instance instead of lambda functions

gccrs: Collect error instance instead of lambda functions

Use error object instead of lambda for error collection.

gcc/rust/ChangeLog:

	* resolve/rust-early-name-resolver-2.0.cc (Early::visit):
	Collect error instead of lambda.
	* resolve/rust-early-name-resolver-2.0.h (std::function<void):
	Remove type alias.
	* rust-diagnostics.h: Change collection type.
	* rust-session-manager.cc (Session::expansion): Change
	collection container.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>