[clang-tools-extra] [clang-tidy] Migrate explicit-constructor check from google to misc and add relative aliases (PR #194807)

[Baranov Victor via cfe-commits]

================
@@ -0,0 +1,54 @@
+.. title:: clang-tidy - misc-explicit-constructor
+
+misc-explicit-constructor
+=========================
+
+Checks that constructors callable with a single argument and conversion
+operators are marked explicit to avoid the risk of unintentional implicit
+conversions.
+
+Consider this example:
+
+.. code-block:: c++
+
+  struct S {
+    int x;
+    operator bool() const { return true; }
+  };
+
+  bool f() {
+    S a{1};
+    S b{2};
+    return a == b;
+  }
+
+The function will return ``true``, since the objects are implicitly converted
+to ``bool`` before comparison, which is unlikely to be the intent.
+
+The check will suggest inserting ``explicit`` before the constructor or
+conversion operator declaration. However, copy and move constructors should not
+be explicit, as well as constructors taking a single ``initializer_list``
+argument.
+
+This code:
+
+.. code-block:: c++
+
+  struct S {
+    S(int a);
+    explicit S(const S&);
+    operator bool() const;
+    ...
+
+will become
+
+.. code-block:: c++
+
+  struct S {
+    explicit S(int a);
+    S(const S&);
+    explicit operator bool() const;
+    ...
+
+
+See https://google.github.io/styleguide/cppguide.html#Explicit_Constructors
----------------
vbvictor wrote:

I think this should be placed in google-specific doc

https://github.com/llvm/llvm-project/pull/194807