[clang-tools-extra] [clang-tidy] add modernize-use-constexpr check (PR #146553)

[via cfe-commits]

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+.. title:: clang-tidy - modernize-use-constexpr
+
+modernize-use-constexpr
+=======================
+
+Finds functions and variables that can be declared 'constexpr'.
+
+The check analyses any function and variable according to the rules defined
+for the language version that the code is compiled with.
+Changing to a newer language standard may therefore offer additional opportunity
+to declare a function or variable as ``constexpr``.
+Furthermore, this check can be incremental in terms of its diagnostics. For
+example, declaring a function ``constepxr`` might create new opportunities of
+marking additional variables or function ``constexpr``, which can only be found
+in subsequent runs of this check.
+
+For variables, the check will only detect variables that can be declared
+``constexpr`` if they are already ``const``.
+This is because this check would have to duplicate the expensive analysis of the
+:doc:`misc-const-correctness<../misc/const-correctness>` check.
+Therefore, it is recommended to have 
+:doc:`misc-const-correctness<../misc/const-correctness>` enabled
+in the Clang-Tidy config when this check is, so that all opportunities for
+``const`` and also ``constexpr`` are explored.
+
+Options
+-------
+
+.. option:: ConservativeLiteralType
+
+  With this option enabled, only literal types that can be constructed at
+  compile-time are considered to supoprt ``constexpr``.
+
+  .. code-block:: c++
+
+    struct NonLiteral{
+      NonLiteral();
+      ~NonLiteral();
+      int &ref;
+    };
+
+  This type is a literal type, but can not be constructed at compile-time.
+  With `ConservativeLiteralType` equal to `true`, variables or funtions
+  with this type are not diagnosed to add ``constexpr``. Default is
+  `true`.
+
+.. option:: AddConstexprToMethodOfClassWithoutConstexprConstructor
+
+  While a function of a class or struct could be declared ``constexpr``, when
+  the class itself can never be constructed at compile-time, then adding
+  ``constexpr`` to a member function is superfluous. This option controls if
+  ``constexpr`` should be added anyways. Default is ``false``.
+
+.. option:: ConstexprString
+
+  The string to use to specify a variable or function as ``constexpr``, for
+  example, a macro. Default is ``constexpr``.
----------------
EugeneZelenko wrote:

```suggestion
  example, a macro. Default is `constexpr`.
```

https://github.com/llvm/llvm-project/pull/146553