gccrs: Fix ICE when hitting invalid types for generics

gccrs: Fix ICE when hitting invalid types for generics

We need to check upfront if the type is valid or not. Then
error with a decent message.

Fixes Rust-GCC#3643
Fixes Rust-GCC#3646
Fixes Rust-GCC#3654
Fixes Rust-GCC#3663
Fixes Rust-GCC#3671

gcc/rust/ChangeLog:

	* resolve/rust-ast-resolve-type.cc (ResolveRelativeTypePath::go): fix error msg
	* typecheck/rust-substitution-mapper.cc (SubstMapper::Resolve): add validation
	(SubstMapper::valid_type): new check
	(SubstMapper::visit): check if can resolve
	* typecheck/rust-substitution-mapper.h: new prototype

gcc/testsuite/ChangeLog:

	* rust/compile/nr2/exclude: nr2 is missing type path error
	* rust/compile/issue-3643.rs: New test.
	* rust/compile/issue-3646.rs: New test.
	* rust/compile/issue-3654.rs: New test.
	* rust/compile/issue-3663.rs: New test.
	* rust/compile/issue-3671.rs: New test.

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