gccrs: libproc_macro: Rename Procmacro subtypes

Rename procedural macro specializations without the "payload" suffix.
Those types will be reused in several places where the old suffix might
be counterintuitive.

libgrust/ChangeLog:

	* libproc_macro/proc_macro.h (struct CustomDerivePayload):
	Rename from CustomDerivePayload to...
	(struct CustomDerive): ... CustomDerive
	(struct AttrPayload): Rename from AttrPayload to ...
	(struct Attribute): ... Attribute
	(struct BangPayload): Rename from BangPayload to ...
	(struct Bang): ... Bang
	(union ProcmacroPayload): Update union with new names.

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