Add CONST_ITEM_MUTATION lint

Fixes #74053
Fixes #55721

This PR adds a new lint `CONST_ITEM_MUTATION`.
Given an item `const FOO: SomeType = ..`, this lint fires on:

* Attempting to write directly to a field (`FOO.field = some_val`) or
  array entry (`FOO.array_field[0] = val`)
* Taking a mutable reference to the `const` item (`&mut FOO`), including
  through an autoderef `FOO.some_mut_self_method()`

The lint message explains that since each use of a constant creates a
new temporary, the original `const` item will not be modified.

compiler/rustc_session/src/lint/builtin.rs

@@ -232,6 +232,12 @@ declare_lint! {
     "detects unaligned references to fields of packed structs",
 }
 
+declare_lint! {
+    pub CONST_ITEM_MUTATION,
+    Warn,
+    "detects attempts to mutate a `const` item",
+}
+
 declare_lint! {
     pub SAFE_PACKED_BORROWS,
     Warn,
@@ -572,6 +578,7 @@ declare_lint_pass! {
         CONST_ERR,
         RENAMED_AND_REMOVED_LINTS,
         UNALIGNED_REFERENCES,
+        CONST_ITEM_MUTATION,
         SAFE_PACKED_BORROWS,
         PATTERNS_IN_FNS_WITHOUT_BODY,
         LATE_BOUND_LIFETIME_ARGUMENTS,