[libcxx] Mark local printf variants as _LIBCPP_HIDDEN to prevent them from becoming part of the DSO.

[EricWF]

Summary

This patch marks 3 symbols as hidden. The symbols are:

1. ZNSt3110sscanf_lEPKcP15__locale_structS1_z
2. _ZNSt3112asprintf_lEPPcP15__locale_structPKcz
3. _ZNSt3112snprintf_lEPcmP15__locale_structPKcz

I used _LIBCPP_HIDDEN as opposed to _LIBCPP_INLINE_VISIBILITY because varargs functions cannot be inlined.

I used sym_check to ensure that these symbols are not already present in the system libraries for OS X 10.10 and FreeBSD 10.1. I don't know of any other platforms that require strict ABI compatibility. Playstation perhaps?

@danalbert: Will this change affect android? Can you check to see if these symbols have been shipped?

On linux these symbols are spuriously emitted into the DSO depending on the optimisation level.

See http://ds2.efcs.ca:8080/builders/abi-checker-debug/builds/5/steps/test.libcxx.abi.release/logs/stdio for an example of this problem.

[mclow.lists]

Could this be solved using the technique proposed in http://reviews.llvm.org/D8652 ?

[danalbert]

@danalbert: Will this change affect android? Can you check to see if these symbols have been shipped?

This looks safe from my end. The sooner the better so that doesn't change :)

This or the internal namespace change sgtm.

[EricWF]

Abandoning in favor of D8652.

include/locale

 #endif
 }
 
-inline
+inline _LIBCPP_HIDDEN
 int __snprintf_l(char *__s, size_t __n, locale_t __l, const char *__format, ...) {
   va_list __va;
   va_start(__va, __format);

   return __res;
 }
 
-inline
+inline _LIBCPP_HIDDEN
 int __asprintf_l(char **__s, locale_t __l, const char *__format, ...) {
   va_list __va;
   va_start(__va, __format);

   return __res;
 }
 
-inline
+inline _LIBCPP_HIDDEN
 int __sscanf_l(const char *__s, locale_t __l, const char *__format, ...) {
   va_list __va;
   va_start(__va, __format);