gccrs: Add async const function ast validation pass

gccrs: Add async const function ast validation pass

Add a check during AST validation pass to ensure functions are either
const or async but not both.

gcc/rust/ChangeLog:

	* checks/errors/rust-ast-validation.cc (ASTValidation::visit): Add
	async const check.

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