gccrs: Fix cast site to not miss type-unifications

gccrs: Fix cast site to not miss type-unifications

When attempting casts we can try a type coercion first, this is a
TryCoercion which will return a result. In the event this is ok we need to
perform a true coercion so that we don't leave missing infeence variable's
ununified.

Fixes #2195

gcc/rust/ChangeLog:

	* typecheck/rust-casts.cc (TypeCastRules::check): do coercion
	(TypeCastRules::emit_cast_error): clang-format

gcc/testsuite/ChangeLog:

	* rust/compile/issue-2195.rs: New test.

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