gccrs: Add name resolution and ast lowering for slice pattern

gccrs: Add name resolution and ast lowering for slice pattern

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-pattern.cc (ASTLoweringPattern::visit):
	Add lowering for SlicePattern.
	* hir/rust-ast-lower-pattern.h: Likewise.
	* resolve/rust-ast-resolve-pattern.cc (PatternDeclaration::visit):
	Add name resolution visit for SlicePattern.
	* resolve/rust-ast-resolve-pattern.h: Likewise.

Signed-off-by: Mahmoud Mohamed <mahadelr19@gmail.com>