gccrs: Add hir lowering to trait item where clauses

Fixes Rust-GCC/gccrs#4828

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-implitem.cc (ASTLowerTraitItem::visit): lower where clauses

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4828.rs: New test.

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