libstdc++: Allow exception classes to move fully-dynamic strings

libstdc++: Allow exception classes to move fully-dynamic strings

The move constructor for the fully-dynamic std::basic_string was not
noexcept until recently, so the std::logic_error and std::runtime_error
move constructors were defined to make non-throwing copies of their
string members, instead of potentially-throwing moves.

Now that move construction is always noexecpt, the exception classes can
always move the string. The fully-dynamic string move assignment was
always noexcept, so I don't know why I special-cased the move assignment
operators of the exception classes. That can be changed too.

libstdc++-v3/ChangeLog:

	* src/c++11/cow-stdexcept.cc [_GLIBCXX_FULY_DYNAMIC_STRING]
	(logic_error, runtime_error): Remove custom definitions.