[libcxx] Export 2 basic_string methods to minimize the effect from internal_linkage switch

[eugenis]

Summary

The switch to internal_linkage attribute removes 3 symbols from the libc++ export table, all in basic_string:
insert(..., InputIterator
insert(..., ForwardIterator
replace(..., InputIterator

These are template methods of a template class. They are instantiated only in functions/methods that are marked with LIBCPP_INLINE_VISIBILITY; normally they are exported as linkonce_odr; after the internal_linkage switch they are not instantiated at all because their callers are never evaluated.

This change explicitly exports these methods to avoid this problem. It does not change the export list of libc++.so in any way.

[EricWF]

Resigning as reviewer. These changes are quite old, and can be redone if needed. In all likelihood I think I would rather remove the symbols from the dylib instead of keep them.

src/string.cpp

 
 template class basic_string<char>;
 template class basic_string<wchar_t>;
 
 template
     string
     operator+<char, char_traits<char>, allocator<char> >(char const*, string const&);
 
+template string::iterator string::insert(string::const_iterator, char const *,
+                                         char const *);
+template basic_string<wchar_t> &
+basic_string<wchar_t>::replace(basic_string<wchar_t>::const_iterator,
+                               basic_string<wchar_t>::const_iterator,
+                               wchar_t const *, wchar_t const *);
+
 namespace
 {
 
 template<typename T>
 inline
 void throw_helper( const string& msg )
 {
 #ifndef _LIBCPP_NO_EXCEPTIONS