gccrs: Handle generic args to traits in paths

Fixes Rust-GCC/gccrs#4853

gcc/rust/ChangeLog:

	* typecheck/rust-substitution-mapper.cc (SubstMapper::valid_type): check for dyn object
	(SubstMapper::visit): substitution on it
	* typecheck/rust-substitution-mapper.h: add impl

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4853.rs: New test.

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