[libc++] Fix libcxx undefined references

[ldionne]

Summary

Related to the commit 31e82037, and it will bring in link error with some
undefined references if Bootstrap Clang with GCC first, such as
_ZNSt3__112basic_stringIwNS_11char_traitsIwEENS_9allocatorIwEEE6appendEPKwm

Here, if you use tool nm to check the libc++.so.1, the above sample function
is showed with attribute t, which is expected attribute W.

[ldionne]

I don't think this is relevant anymore since D103964.

[Allen]

thanks, D103964 seems a more complete solution include this change, so this can be closed

[ldionne]

Commandeering to abandon.

libcxx/include/__config

 #elif _LIBCPP_DEBUG == 0
 # define _LIBCPP_DEBUG_LEVEL 1
 #elif _LIBCPP_DEBUG == 1
 # define _LIBCPP_DEBUG_LEVEL 2
 #else
 # error Supported values for _LIBCPP_DEBUG are 0 and 1
 #endif
 
-// _LIBCPP_DEBUG_LEVEL is always defined to one of [0, 1, 2] at this point
-#if _LIBCPP_DEBUG_LEVEL >= 1 && !defined(_LIBCPP_DISABLE_EXTERN_TEMPLATE)
-# define _LIBCPP_EXTERN_TEMPLATE(...)
-#endif
-
 #ifdef _LIBCPP_DISABLE_EXTERN_TEMPLATE
 # define _LIBCPP_EXTERN_TEMPLATE(...)
 # define _LIBCPP_EXTERN_TEMPLATE_EVEN_IN_DEBUG_MODE(...)
 #endif
 
 #ifndef _LIBCPP_EXTERN_TEMPLATE
 #define _LIBCPP_EXTERN_TEMPLATE(...) extern template __VA_ARGS__;
 #endif