tests: add for_range_signed test

Author: Firestar99

tests/ui/lang/control_flow/for_range.rs

@@ -3,6 +3,6 @@
 use spirv_std::spirv;
 
 #[spirv(fragment)]
-pub fn main(#[spirv(flat)] i: i32) {
+pub fn main(#[spirv(flat)] i: u32) {
     for _ in 0..i {}
 }

tests/ui/lang/control_flow/for_range_signed-broken.rs

@@ -0,0 +1,13 @@
+// build-fail
+// normalize-stderr-test "\S*/library/core/src/" -> "$$CORE_SRC/"
+
+// FIXME(eddyb) this should work, but `unchecked_add_unsigned` doesn't exist,
+// so range internals use `a.checked_add_unsigned(b).unwrap_unchecked()` instead,
+// and that requires working `overflowing_add`, which we don't yet have.
+
+use spirv_std::spirv;
+
+#[spirv(fragment)]
+pub fn main(#[spirv(flat)] i: i32) {
+    for _ in 0..i {}
+}