add implicit constructor for `vector<bool>::const_reference` for libcpp

Summary:
Despite the standard, `std::vector<bool>::const_reference` is not `bool` in
libcpp: http://howardhinnant.github.io/onvectorbool.html

Add the implicit ctor so that `f(dynamic)` can be invoked with `f(v[idx])`
where `v` is a `const vector<bool>` under libc++.

Reviewed By: ot

Differential Revision: D8992805

fbshipit-source-id: 0675174c2a247257238bb11b2c7b319653fe92a3

folly/dynamic-inl.h

@@ -364,6 +364,10 @@ struct dynamic::NumericTypeHelper<double> {
 
 inline dynamic::dynamic(std::vector<bool>::reference b)
     : dynamic(static_cast<bool>(b)) {}
+#if defined(_LIBCPP_VERSION)
+inline dynamic::dynamic(std::vector<bool>::const_reference b)
+    : dynamic(static_cast<bool>(b)) {}
+#endif
 
 template <
     class T,

folly/dynamic.h

@@ -177,8 +177,14 @@ struct dynamic : private boost::operators<dynamic> {
    * bool. Calling a function f(dynamic) with f(v[idx]) would require a double
    * implicit conversion (reference -> bool -> dynamic) which is not allowed,
    * hence we explicitly accept the reference proxy.
+   *
+   * std::vector<bool>::const_reference is not bool in libcpp:
+   * http://howardhinnant.github.io/onvectorbool.html
    */
   /* implicit */ dynamic(std::vector<bool>::reference val);
+#if defined(_LIBCPP_VERSION)
+  /* implicit */ dynamic(std::vector<bool>::const_reference val);
+#endif
 
   /*
    * Create a dynamic that is an array of the values from the supplied