Rollup merge of rust-lang#128666 - pitaj:intrinsic-overflow_checks, r=BoxyUwU

Add `overflow_checks` intrinsic

This adds an intrinsic which allows code in a pre-built library to inherit the overflow checks option from a crate depending on it. This enables code in the standard library to explicitly change behavior based on whether `overflow_checks` are enabled, regardless of the setting used when standard library was compiled.

This is very similar to the `ub_checks` intrinsic, and refactors the two to use a common mechanism.

The primary use case for this is to allow the new `RangeFrom` iterator to yield the maximum element before overflowing, as requested [here](rust-lang#125687 (comment)). This PR includes a working `IterRangeFrom` implementation based on this new intrinsic that exhibits the desired behavior.

[Prior discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Ability.20to.20select.20code.20based.20on.20.60overflow_checks.60.3F)

Author: Zalathar

core/src/intrinsics/mod.rs

@@ -2589,6 +2589,24 @@ pub const fn ub_checks() -> bool {
     cfg!(ub_checks)
 }
 
+/// Returns whether we should perform some overflow-checking at runtime. This eventually evaluates to
+/// `cfg!(overflow_checks)`, but behaves different from `cfg!` when mixing crates built with different
+/// flags: if the crate has overflow checks enabled or carries the `#[rustc_inherit_overflow_checks]`
+/// attribute, evaluation is delayed until monomorphization (or until the call gets inlined into
+/// a crate that does not delay evaluation further); otherwise it can happen any time.
+///
+/// The common case here is a user program built with overflow_checks linked against the distributed
+/// sysroot which is built without overflow_checks but with `#[rustc_inherit_overflow_checks]`.
+/// For code that gets monomorphized in the user crate (i.e., generic functions and functions with
+/// `#[inline]`), gating assertions on `overflow_checks()` rather than `cfg!(overflow_checks)` means that
+/// assertions are enabled whenever the *user crate* has overflow checks enabled. However if the
+/// user has overflow checks disabled, the checks will still get optimized out.
+#[inline(always)]
+#[rustc_intrinsic]
+pub const fn overflow_checks() -> bool {
+    cfg!(debug_assertions)
+}
+
 /// Allocates a block of memory at compile time.
 /// At runtime, just returns a null pointer.
 ///