`basic_regex` is missing move operations

[eisenwave]

regex/include/boost/regex/v5/basic_regex.hpp

Lines 370 to 377 in ed6ebbd

	basic_regex& operator=(const basic_regex& that)
	{
	return assign(that);
	}
	basic_regex& operator=(const charT* ptr)
	{
	return assign(ptr);
	}

Notice that while there is a copy constructor and copy assignment operator, there are no move operations. This is bad for two reasons:

1. Clangd may give the user a warning when they apply std::move to a type, but no move is actually happening. If the user decides not to std::move, they forever pessimize and do not opt into a move constructor added by Boost in the future.

2. Internally, basic_regex is wrapping a std::shared_ptr to some implementation. std::shared_ptr significantly benefits from move operations because it allows for stealing the pointer within another smart pointer; the reference counter is never updated. It is somewhat wasteful not to std::move a std::shared_ptr when it's possible, even if copying remains a shallow copy.