Add flag _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATIONS for basic_string ABI

[mvels]

Summary

This review is a mostly trivial change to use an explicit ABI flag for the unstable external template list. This follows the practice for an ABI flag per feature, and provides a spot for the rational / motivation for the flag.

[mvels]

Heads up: this trivial change was per-approved per EricWf, I will be committing after double checking build and unit tests for stable and unstable

libcxx/include/__config

 #  define _LIBCPP_ABI_OPTIMIZED_LOCALE_NUM_GET
 // Use the smallest possible integer type to represent the index of the variant.
 // Previously libc++ used "unsigned int" exclusively.
 #  define _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION
 // Unstable attempt to provide a more optimized std::function
 #  define _LIBCPP_ABI_OPTIMIZED_FUNCTION
 // All the regex constants must be distinct and nonzero.
 #  define _LIBCPP_ABI_REGEX_CONSTANTS_NONZERO
+// Re-worked external template instantiations for std::string with a focus on
+// performance and fast-path inlining.
+#  define _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATION
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !defined(_LIBCPP_OBJECT_FORMAT_COFF)
 // Enable compiling copies of now inline methods into the dylib to support
 // applications compiled against older libraries. This is unnecessary with
 // COFF dllexport semantics, since dllexport forces a non-inline definition
 // of inline functions to be emitted anyway. Our own non-inline copy would
 // conflict with the dllexport-emitted copy, so we disable it.
 #    define _LIBCPP_DEPRECATED_ABI_LEGACY_LIBRARY_DEFINITIONS_FOR_INLINE_FUNCTIONS

libcxx/include/string

 basic_string<_CharT, _Traits, _Allocator>::__subscriptable(const const_iterator* __i, ptrdiff_t __n) const
 {
     const value_type* __p = _VSTD::__to_address(__i->base()) + __n;
     return this->data() <= __p && __p < this->data() + this->size();
 }
 
 #endif  // _LIBCPP_DEBUG_LEVEL >= 2
 
-#if defined(_LIBCPP_ABI_UNSTABLE) || _LIBCPP_ABI_VERSION >= 2
+#ifdef _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATION
 _LIBCPP_STRING_UNSTABLE_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE, char)
 _LIBCPP_STRING_UNSTABLE_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE, wchar_t)
 #else
 _LIBCPP_STRING_V1_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE, char)
 _LIBCPP_STRING_V1_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE, wchar_t)
 #endif
 
 #if _LIBCPP_STD_VER > 11

libcxx/src/string.cpp

 #include "stdexcept"
 #include <stdio.h>
 #include "__debug"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS __basic_string_common<true>;
 
-#if defined(_LIBCPP_ABI_UNSTABLE) || _LIBCPP_ABI_VERSION >= 2
+#ifdef _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATION
 _LIBCPP_STRING_UNSTABLE_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, char)
 _LIBCPP_STRING_UNSTABLE_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, wchar_t)
 #else
 _LIBCPP_STRING_V1_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, char)
 _LIBCPP_STRING_V1_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, wchar_t)
 #endif
 
 template