gccrs: Used `IndexVec` for BasicBlocks

gccrs: Used `IndexVec` for BasicBlocks

gcc/rust/ChangeLog:

	* checks/errors/borrowck/rust-bir-builder-expr-stmt.cc
	(ExprStmtBuilder::visit): Use strong type as index and
	remove access to numeric value.
	* checks/errors/borrowck/rust-bir-builder-internal.h
	(struct BuilderContext): Likewise.
	* checks/errors/borrowck/rust-bir-dump.cc (simplify_cfg):
	Likewise.
	(Dump::go): Likewise.
	(Dump::visit): Likewise.
	* checks/errors/borrowck/rust-bir-fact-collector.h
	(class FactCollector): Likewise.
	(points): Likewise.
	* checks/errors/borrowck/rust-bir.h (struct BasicBlockId): Used
	IndexVec for BasicBlocks.
	(struct Function): Likewise.
	* checks/errors/borrowck/rust-borrow-checker-diagnostics.cc
	(BorrowCheckerDiagnostics::get_statement): Change the extracted
	index to strong type.

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