gccrs: Remove solve_missing_mappings_from_this to handle covariants

change how we monomorphize dyn-items when we need to compute the generics

We might have a trait item such as:

  impl<'a, T> FnLike<&'a T, &'a T> for Identity {
      fn call(&self, arg: &'a T) -> &'a T { ... }
  }

Which ended up monomorphized badly to:

  const isize & const &
    <example::Identity as example::FnLike::<& T, & T>>::call<& isize>
    (const struct example::Identity & const self,
     const isize & const & const arg)

This is wrong because it turned into a double reference type becasuse this
bug was consistent bugs were not picked up but this is not correct. We now
reuse our type inference infrastructure to solve the parameters instead.

Fixes #1984

gcc/rust/ChangeLog:

	* backend/rust-compile.cc: use unify_and instead
	* typecheck/rust-tyty-subst.cc (SubstitutionRef::solve_missing_mappings_from_this): remove
	* typecheck/rust-tyty-subst.h: update header

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

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