Add regression test for #70155.

With #70166 merged, `RangeInclusive` now derives `PartialEq` and `Eq`, implementing structural equality and as a side effect the range is now usable with const generics, closing #70155.

A test is added to avoid a change to the private fields or the equality implementation of the range from subtly reverting #70155.

src/test/ui/const-generics/issues/issue-70155.rs

@@ -0,0 +1,12 @@
+// check-pass
+#![allow(incomplete_features)]
+#![feature(const_generics)]
+
+// Regression test for #70155:
+// `RangeInclusive` should be usable with const generics
+
+struct S<const R: std::ops::RangeInclusive<usize>>;
+
+const C : S<{ 0 ..= 999 }> = S;
+
+pub fn main() {}