gccrs: Fix narrowing conversion warnings

gccrs: Fix narrowing conversion warnings

Fixes PR#119641

gcc/rust/ChangeLog:

	* checks/errors/borrowck/rust-bir-place.h
	(IndexVec::size_type): Add.
	(IndexVec::MAX_INDEX): Add.
	(IndexVec::size): Change the return type to the type of the
	internal value used by the index type.
	(PlaceDB::lookup_or_add_variable): Use the return value from the
	PlaceDB::add_place call.
	* checks/errors/borrowck/rust-bir.h
	(struct BasicBlockId): Move this definition before the
	definition of the struct Function.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>