gccrs: Track fn_once output lang item properly

In order to setup the Output assoicated type we can rely on using generic
argument bindings. So for example when we have the FnOnce trait:

  #[lang = "fn_once"]
  pub trait FnOnce<Args> {
      #[lang = "fn_once_output"]
      type Output;

      extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
  }

Thn we might have a function such as:

  pub fn map<R, F: FnOnce(T) -> R>(self, f: F) -> Option<R> { ... }

This trait bound predicate of FnOnce(T) -> R we setup generics for the
bound as:

  FnOnce<(T), Output=R>

This means we can reuse our generic arguments handling to get this support.

Fixes #2105

gcc/rust/ChangeLog:

	* typecheck/rust-tyty-bounds.cc (TypeCheckBase::get_predicate_from_bound): track output
	* util/rust-hir-map.cc (Mappings::lookup_trait_item_lang_item): new helper
	* util/rust-hir-map.h: add prototype for helper

gcc/testsuite/ChangeLog:

	* rust/compile/issue-2105.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

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