[clang-tools-extra] Add bugprone-loop-variable-copied-then-modified clang-tidy check. (PR #157213)

[Baranov Victor via cfe-commits]

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@@ -0,0 +1,48 @@
+.. title:: clang-tidy - bugprone-loop-variable-copied-then-modified
+
+bugprone-loop-variable-copied-then-modified
+===========================================
+
+Detects when a loop variable is copied and then subsequently modified and
+suggests replacing with a reference or an explicit copy.
+
+This pattern is considered bugprone because, frequently, programmers do not
+realize that they are modifying a *copy* rather than an underlying value,
+resulting in subtly erroneous code.
+
+For instance, the following code attempts to null out a value in a map, but only
+succeeds in nulling out a value in a *copy* of the map:
+
+.. code-block:: c++
+
+  for (auto target : target_map) {
+    target.value = nullptr;
+  }
+
+The programmer is likely to have intended this code instead:
+
+.. code-block:: c++
+
+  for (auto& target : target_map) {
+    target.value = nullptr;
+  }
+
+This warning can be suppressed in one of two ways:
+  - In cases where the programmer did not intend to create a copy, they can
+    convert the loop variable to a reference or a ``const`` reference. A fix-it
+    message will provide a naive suggestion of how to achieve this, which works
+    in most cases.
----------------
vbvictor wrote:

If the fix-it works "most of the time" it should be a "fix-note" that will only be applied when `clang-tidy` is invoked with `--fix-notes`. See how other checks implement it, it must be a small change to `Diag()`.

https://github.com/llvm/llvm-project/pull/157213