gccrs: ast: Add manual dispatch for meta item downcasting

gccrs: ast: Add manual dispatch for meta item downcasting

Several meta items shall be downcasted in various places, this commit
introduces values to dispatch on.

gcc/rust/ChangeLog:

	* ast/rust-ast.h (class MetaItem): Add MetaItem dispatch values.
	* ast/rust-expr.h: Add LitExpr and PathLit dispatch.
	* ast/rust-macro.h: Add remaining dispatch getters.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>