gccrs: libproc_macro: Add helpers to construct Procmacro

gccrs: libproc_macro: Add helpers to construct Procmacro

Add some helper functions to create Procmacro tagged unions.

libgrust/ChangeLog:

	* libproc_macro/proc_macro.cc (Procmacro::make_derive):
	Add custom derive macro constructor.
	(Procmacro::make_attribute): Add attribute macro constructor.
	(Procmacro::make_bang): Add bang macro constructor.
	* libproc_macro/proc_macro.h (struct Procmacro):
	Add helper function prototypes.

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