Strong type ScopeId

Strong type ScopeId

gcc/rust/ChangeLog:

	* checks/errors/borrowck/rust-bir-builder-expr-stmt.cc
	(ExprStmtBuilder::setup_loop): Use value of ScopeId.
	(ExprStmtBuilder::visit): Use continue scope id instead of
	continue basic block id.
	* checks/errors/borrowck/rust-bir-builder-internal.h: Use value
	of ScopeId.
	* checks/errors/borrowck/rust-bir-dump.cc (Dump::go): Use
	ROOT_VALUE instead of hardcoded 0.
	(Dump::visit_scope): Use value of ScopeId.
	* checks/errors/borrowck/rust-bir-place.h (struct ScopeId):
	ScopeId is now a struct.
	(std::numeric_limits::max): Set invalid ScopeId.

Signed-off-by: Kushal Pal <kushalpal109@gmail.com>