Auto merge of #130540 - veera-sivarajan:fix-87525, r=estebank

Add a Lint for Pointer to Integer Transmutes in Consts

Fixes #87525

This PR adds a MirLint for pointer to integer transmutes in const functions and associated consts. The implementation closely follows this comment: https://github.com/rust-lang/rust/pull/85769#issuecomment-880969112. More details about the implementation can be found in the comments.

Note: This could break some sound code as mentioned by RalfJung in https://github.com/rust-lang/rust/pull/85769#issuecomment-886491680:

> ... technically const-code could transmute/cast an int to a ptr and then transmute it back and that would be correct -- so the lint will deny some sound code. Does not seem terribly likely though.

References:
1. https://doc.rust-lang.org/std/mem/fn.transmute.html
2. https://doc.rust-lang.org/reference/items/associated-items.html#associated-constants

compiler/rustc_mir_transform/src/errors.rs

@@ -121,3 +121,10 @@ pub(crate) struct MustNotSuspendReason {
     pub span: Span,
     pub reason: String,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(mir_transform_undefined_transmute)]
+#[note]
+#[note(mir_transform_note2)]
+#[help]
+pub(crate) struct UndefinedTransmute;