gccrs: Generic pointers are coerceable

This is a complex type-system change where it begins out journey to get
rid of our can_eq interface. Rust allows:

  let x:*mut T
  let y = x as *mut u8;

Which requires us to consider find a way to infer what T should be so as
to keep unify happy. This means we need to introduce a new unify_and
interface where we can optionally inject inference variables as well as
only commit the inference variable joins when they are sucsessful.

So for this case we can then inject an implicit inference variables for T
that can unify against u8 to make this a valid type-resolution.

Fixes #1930

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

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::resolve_method_address): update to new inteface
	* typecheck/rust-coercion.cc (TypeCoercionRules::coerce_unsafe_ptr): likewise
	(TypeCoercionRules::coerce_borrowed_pointer): likewise
	* typecheck/rust-hir-type-check.h: likewise
	* typecheck/rust-type-util.cc (unify_site_and): new interface to allow for infer and commit
	* typecheck/rust-type-util.h (unify_site_and): likewise
	* typecheck/rust-typecheck-context.cc (TypeCheckContext::clear_type): new interface
	* typecheck/rust-unify.cc (UnifyRules::UnifyRules): update
	(UnifyRules::Resolve): new optional flags for commit and infer
	(UnifyRules::go): likewise
	(UnifyRules::expect_adt): refactor to use new interface
	(UnifyRules::expect_reference): likewise
	(UnifyRules::expect_pointer): likewise
	(UnifyRules::expect_array): likewise
	(UnifyRules::expect_slice): likewise
	(UnifyRules::expect_fndef): likewise
	(UnifyRules::expect_fnptr): likewise
	(UnifyRules::expect_tuple): likewise
	(UnifyRules::expect_closure): likewise
	* typecheck/rust-unify.h: refactor interface

gcc/testsuite/ChangeLog:

	* rust/compile/issue-1930.rs: New test.

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