gccrs: libproc_macro: Change Ident structure

gccrs: libproc_macro: Change Ident structure

Use FFIString in Ident structure rather that a raw pointer and a
length, this will reduce the size of the code dealing with raw
pointers. Which should prevent some error.

gcc/rust/ChangeLog:

	* util/rust-token-converter.cc (from_ident): Adapt code to new
	constructor.

libgrust/ChangeLog:

	* libproc_macro/ident.cc (Ident__new): Constructor
	accepts an FFIString now.
	(Ident__new_raw): Likewise.
	(Ident::clone): Internal members change means clone also change.
	(Ident::make_ident): Change constructor call.
	(Ident::drop): Add call to FFIString::clone.
	* libproc_macro/ident.h (struct Ident): Remove raw
	pointer and length, add an FFIString inside instead.
	(Ident__new): Change constructor.
	(Ident__new_raw): Change constructor.

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