Work around Clang bug 22354.

https://llvm.org/bugs/show_bug.cgi?id=22354

At the time DisallowConstCopy was introduced, GCC 4.7 and Clang 3.2 refused to allow the non-const copy constructors to be defaulted inline. This must have been concluded to be a compiler bug, as GCC 4.8 and Clang 3.4 seem fine with it. So, we can clean this up.

Meanwhile, the above-linked Clang bug triggered when a DisallowConstCopy derivative is captured by value in a C++14 generic lambda. capnp::CallContext in particular tended to be hit by this a lot. The bug only affects non-default (or out-of-line default) copy constructors, and thus is conveniently sidestepped by this change.

c++/src/kj/common.h

@@ -349,16 +349,12 @@ struct DisallowConstCopy {
   // disallow const copies.  Hey, cool, that's exactly what we want.
 
   DisallowConstCopy() = default;
-  DisallowConstCopy(DisallowConstCopy&);
+  DisallowConstCopy(DisallowConstCopy&) = default;
   DisallowConstCopy(DisallowConstCopy&&) = default;
-  DisallowConstCopy& operator=(DisallowConstCopy&);
+  DisallowConstCopy& operator=(DisallowConstCopy&) = default;
   DisallowConstCopy& operator=(DisallowConstCopy&&) = default;
 };
 
-// Apparently these cannot be defaulted inside the class due to some obscure C++ rule.
-inline DisallowConstCopy::DisallowConstCopy(DisallowConstCopy&) = default;
-inline DisallowConstCopy& DisallowConstCopy::operator=(DisallowConstCopy&) = default;
-
 template <typename T>
 struct DisallowConstCopyIfNotConst: public DisallowConstCopy {
   // Inherit from this when implementing a template that contains a pointer to T and which should