[clang-tools-extra] [clang-tidy] add modernize-use-constexpr check (PR #162741)

[Congcong Cai 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``.
+
+This check currently supports the ``constexpr`` rule-set of C++11.
+
+The check analyses any function and variable according to the rules defined
+for the language version that the code compiles with.
+Changing to a newer language standard may therefore offer additional
+opportunities 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.
+
+Before C++23, ``static constexpr`` variables could not be declared inside a
+``constexpr`` function. This check prefers adding ``constexpr`` to an enclosing
+function over adding ``constexpr`` to a static local variable inside that
+function.
+
+Limitations
+-----------
+
+* Only analyzes variables declared ``const``, because this check would have
+  to duplicate the expensive analysis of the 
+  :doc:`misc-const-correctness<../misc/const-correctness>` check.
+  For the best results, enable both `misc-const-correctness` and
+  `modernize-use-constexpr` together.
+
+* Only analyzes variable declarations that declare a single variable
+
+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;
+    };
----------------
HerrCai0907 wrote:

Only struct with constexpr constructor can be marked as constexpr. even `ConservativeLiteralType` is false, `NonLiteral` cannot be marked as constexpr IMO.
This code block will confuse me how to make this `NonLiteral` constexpr.

https://github.com/llvm/llvm-project/pull/162741