gccrs: Desugar double borrows into two HIR:BorrowExpr's

gccrs: Desugar double borrows into two HIR:BorrowExpr's

We simply hit a gcc_unreachable() on double borrows but it seems reasonable
to just desugar the AST into a borrow of a borrow to foo. Instead of a
borrow expression with a flag to be respected.

Fixes #1506

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-expr.h: Lower double borrow expressions to two
	`HIR::BorrowExpr`s.
	* hir/tree/rust-hir-expr.h: Remove `is_double_borrow` field from
	`HIR::BorrowExpr`.
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit): Remove
	call to `gcc_unreachable` on double borrow expressions.

gcc/testsuite/ChangeLog:

	* rust/compile/torture/issue-1506.rs: New test.