gccrs: Emit error diagnostic for bad impl type usage

gccrs: Emit error diagnostic for bad impl type usage

Rust only allows impl traits to be used in the return position of
functions.

Fixes Rust-GCC#1485

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-implitem.cc (ASTLowerImplItem::visit): allow impl type
	* hir/rust-ast-lower-item.cc (ASTLoweringItem::visit): likewise
	* hir/rust-ast-lower-type.cc (ASTLoweringType::ASTLoweringType): new flag for impl trait
	(ASTLoweringType::translate): pass flag
	(ASTLoweringType::visit): track impl trait tag
	(ASTLoweringType::emit_impl_trait_error): new diagnostic
	* hir/rust-ast-lower-type.h: add new field

gcc/testsuite/ChangeLog:

	* rust/compile/impl_trait_diag.rs: New test.
	* rust/compile/issue-1485.rs: New test.

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