gccrs: converter: Return a vector to const pointers

gccrs: converter: Return a vector to const pointers

We do not need mutability on the output vector. Also add an accumulator
for punct tokens.

gcc/rust/ChangeLog:

	* util/rust-token-converter.cc (from_tokenstream): Add vector
	for joined punct accumulation.
	(from_ident): Accept const pointer vector.
	(from_literal): Likewise.
	(from_punct): Likewise.
	(from_group): Likewise.
	(from_tokentree): Likewise.
	(convert): Likewise.
	* util/rust-token-converter.h (convert): Likewise.

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