gccrs: ast: Add DesugarForLoop class

gccrs: ast: Add DesugarForLoop class

gcc/rust/ChangeLog:

	* ast/rust-desugar-for-loops.cc: New file.
	* ast/rust-desugar-for-loops.h: New file.
	* hir/rust-ast-lower-expr.cc (ASTLoweringExpr::visit): Make lowering of for-loops an
	unreachable.
	* Make-lang.in: Compile it.

gcc/testsuite/ChangeLog:

	* rust/compile/for-loop1.rs: New test.
	* rust/compile/for-loop2.rs: New test.
	* rust/execute/torture/for-loop1.rs: New test.
	* rust/execute/torture/for-loop2.rs: New test.
	* rust/compile/nr2/exclude: Exclude for-loop1.rs