Fix for clang, when adapted type is convertible from other types.

The problem was with copy/move constructors of boost::optional<T>, which invoked
optional_base<T> constructors with a single argument. Since optional_base has
copy/move constructors along with initializing constructors taking a single
argument, the latter may be considered by the compiler for viability. While
doing so, the compiler may instantiate the template constructor of T with
an argument of optional_base<T>, which in turn may fail if the constructor
attempts to inspect the type of its argument (e.g. to constrain the set of
acceptable types). Specifically, this happens with clang in C++03 mode,
when boost::multiprecision::number is wrapped in boost::optional and
a copy constructor of boost::optional is invoked.

This commit fixes the problem by destinguishing copy/move constructors of
optional_base from initializing constructors with an additional tag argument.

include/boost/optional/detail/optional_trivially_copyable_base.hpp

@@ -30,7 +30,7 @@ class tc_optional_base : public optional_tag
       :
       m_initialized(false) {}
 
-    tc_optional_base ( argument_type val )
+    tc_optional_base ( init_value_tag, argument_type val )
       :
       m_initialized(true), m_storage(val) {}