Use default visibility to work around clang -fvisibility-inlines-hidden bug

Clang (including trunk and many older versions) incorrectly marks static
local variables (__tag) hidden when -fvisibility-inlines-hidden is used.
This can lead to multiple instances of __tag when shares objects are used.

2018-07-20  Fangrui Song  <maskray@google.com>

	* include/bits/shared_ptr_base.h (_Sp_make_shared_tag::_S_ti): Use
	_GLIBCXX_VISIBILITY(default).

From-SVN: r262903

libstdc++-v3/ChangeLog

+2018-07-20  Fangrui Song  <maskray@google.com>
+
+	* include/bits/shared_ptr_base.h (_Sp_make_shared_tag::_S_ti): Use
+	_GLIBCXX_VISIBILITY(default).
+
 2018-07-20  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/86603

libstdc++-v3/include/bits/shared_ptr_base.h

@@ -508,7 +508,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       friend class _Sp_counted_ptr_inplace;
 
     static const type_info&
-    _S_ti() noexcept
+    _S_ti() noexcept _GLIBCXX_VISIBILITY(default)
     {
       alignas(type_info) static constexpr _Sp_make_shared_tag __tag;
       return reinterpret_cast<const type_info&>(__tag);