gccrs: ast: Add outer attribute getter to Expr class

gccrs: ast: Add outer attribute getter to Expr class

We need to retrieve outer attributes from some Expressions depending on
their context. This means this should be retrieved from their parent
node. But expr did not have a getter for outer attributes since some
expr can't have any outer attribute.

gcc/rust/ChangeLog:

	* ast/rust-ast.h: Add getter to Expr class.
	* ast/rust-expr.h: Add override attribute to existing getters.
	Also implement it for RangeExpr, attempting to retrieve outer
	attributes on those types will crash the compiler.
	* ast/rust-macro.h: Add override attribute to existing getters.
	* ast/rust-path.h: Likewise.

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