[NFC] Rename _LIBCPP_TYPE_VIS_ONLY to _LIBCPP_TEMPLATE_VIS

The name _LIBCPP_TYPE_VIS_ONLY is no longer accurate because both
_LIBCPP_TYPE_VIS and _LIBCPP_TYPE_VIS_ONLY expand to
__attribute__((__type_visibility__)) with Clang. The only remaining difference
is that _LIBCPP_TYPE_VIS_ONLY can be applied to templates whereas
_LIBCPP_TYPE_VIS cannot (due to dllimport/dllexport not being allowed on
templates).

This patch renames _LIBCPP_TYPE_VIS_ONLY to _LIBCPP_TEMPLATE_VIS.

llvm-svn: 291035
diff --git a/libcxx/include/initializer_list b/libcxx/include/initializer_list
index 663e49b..d6dfa80 100644
--- a/libcxx/include/initializer_list
+++ b/libcxx/include/initializer_list
@@ -56,7 +56,7 @@
 #ifndef _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS
 
 template<class _Ep>
-class _LIBCPP_TYPE_VIS_ONLY initializer_list
+class _LIBCPP_TEMPLATE_VIS initializer_list
 {
     const _Ep* __begin_;
     size_t    __size_;