gccrs: Add rest pattern support for AST::SlicePattern

gccrs: Add rest pattern support for AST::SlicePattern

The main change can be found in ast/rust-pattern.h, which introduces 2 item types for
AST::SlicePattern - one without rest pattern (SlicePatternItemsNoRest) & the other with
it (SlicePatternItemsHasRest). This led to a number of cascading changes as seen in the
changelog.

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc: Add support for the 2 new classes.
	* ast/rust-ast-collector.h: Header file update for above.
	* ast/rust-ast-full-decls.h: Add forward decls for the 2 new classes.
	* ast/rust-ast-visitor.cc: Add visit support for the 2 new classes.
	* ast/rust-ast-visitor.h: Header file update for above.
	* ast/rust-pattern.cc: Implementation of certain methods for the 2 new classes.
	* ast/rust-pattern.h: Define the 2 new classes. Update SlicePattern to be able to hold
	2 kinds of items - SlicePatternItemsNoRest or SlicePatternItemsRest.
	* expand/rust-cfg-strip.cc: Add support for the 2 new classes.
	* expand/rust-cfg-strip.h: Header file update for above.
	* expand/rust-derive.h: Add visits for the 2 new classes.
	* hir/rust-ast-lower-base.cc: Add visits for the 2 new classes.
	* hir/rust-ast-lower-base.h: Header file update for above.
	* hir/rust-ast-lower-pattern.cc: Update lowering of SlicePattern to support
	SlicePatternItemsNoRest.
	* parse/rust-parse-impl.h (parse_slice_pattern()): Add support for parsing DOT_DOT into
	respective SlicePatternItems.
	* resolve/rust-ast-resolve-base.cc: Add visits for the 2 new classes.
	* resolve/rust-ast-resolve-base.h: Header file update for above.
	* resolve/rust-ast-resolve-pattern.cc: Update SlicePattern resolution to support new
	classes.

Signed-off-by: Yap Zhi Heng <yapzhhg@gmail.com>