[clang-tools-extra] [clang-tidy] Add `modernize-use-bit-cast` check (PR #189962)

[via cfe-commits]

================
@@ -0,0 +1,64 @@
+.. title:: clang-tidy - modernize-use-bit-cast
+
+modernize-use-bit-cast
+======================
+
+Finds conservative object-to-object ``memcpy`` type punning that can be
+rewritten as ``std::bit_cast`` in C++20 and later.
+
+The check targets the common pattern of copying the full object representation
+of one trivially copyable object into another trivially copyable object of a
+different type:
+
+.. code-block:: c++
+
+  float src = 1.0f;
+  unsigned int dst;
+  std::memcpy(&dst, &src, sizeof(src));
+
+This is rewritten to:
+
+.. code-block:: c++
+
+  float src = 1.0f;
+  unsigned int dst;
+  dst = std::bit_cast<unsigned int>(src);
+
+The fix intentionally replaces only the ``memcpy`` call. It does not fold a
+preceding declaration into ``auto dst = ...`` because doing so can change the
+construction behavior of the destination object.
+
+It only matches direct named source and destination objects, or direct
+field subobjects accessed through ``.``, ``->``, ``.*``, or ``->*``,
+and only when:
+
+* both object types are trivially copyable,
+* neither object type is a pointer, array, function type, or
+  volatile-qualified,
+* the source and destination types differ,
+* the copy size is expressed as ``sizeof(...)`` for either copied type, and
+* the ``memcpy`` call appears in a discarded-value context, such as a statement
+  body, the operand of an explicit ``(void)`` cast, or a comma subexpression
+  whose value is discarded.
+
+The check intentionally does not diagnose:
+
+* pointer punning,
+* array or buffer manipulation,
+* macro expansions,
+* dependent template cases,
+* unevaluated contexts such as ``sizeof(memcpy(...))``,
+* larger expressions where the ``memcpy`` value affects the enclosing
+  expression, such as conditions or operands of unrelated operators,
+* calls where the return value of ``memcpy`` is used, or
+* unrelated overloads such as a user-defined ``memcpy``.
+
+If needed, the fix also inserts ``#include <bit>``.
+
+Options
+-------
+
+.. option:: IncludeStyle
+
+   A string specifying which include-style is used, ``llvm`` or ``google``.
----------------
EugeneZelenko wrote:

```suggestion
   A string specifying which include-style is used, `llvm` or `google`.
```

https://github.com/llvm/llvm-project/pull/189962