[libc++] Replace uses of _LIBCPP_ALWAYS_INLINE by _LIBCPP_INLINE_VISIBILITY

Summary:
We never actually mean to always inline a function -- all the uses of
the macro I could find are actually attempts to control the visibility
of symbols. This is better described by _LIBCPP_INLINE_VISIBILITY, which
is actually always defined the same.

This change is orthogonal to the decision of what we're actually going
to do with _LIBCPP_INLINE_VISIBILITY -- it just simplifies things by
having one canonical way of doing things.

Reviewers: EricWF

Subscribers: christof, llvm-commits, dexonsmith, erikvanderpoel, mclow.lists

Differential Revision: https://reviews.llvm.org/D48892

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@336369 91177308-0d34-0410-b5e6-96231b3b80d8

include/ostream

@@ -232,7 +232,7 @@ public:
     basic_ostream& seekp(off_type __off, ios_base::seekdir __dir);
 
 protected:
-    _LIBCPP_ALWAYS_INLINE
+    _LIBCPP_INLINE_VISIBILITY
     basic_ostream() {}  // extension, intentially does not initialize
 };
 
@@ -249,7 +249,7 @@ public:
     explicit sentry(basic_ostream<_CharT, _Traits>& __os);
     ~sentry();
 
-    _LIBCPP_ALWAYS_INLINE
+    _LIBCPP_INLINE_VISIBILITY
         _LIBCPP_EXPLICIT
         operator bool() const {return __ok_;}
 };