[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/vector

@@ -295,7 +295,7 @@ template <bool>
 class __vector_base_common
 {
 protected:
-    _LIBCPP_ALWAYS_INLINE __vector_base_common() {}
+    _LIBCPP_INLINE_VISIBILITY __vector_base_common() {}
     _LIBCPP_NORETURN void __throw_length_error() const;
     _LIBCPP_NORETURN void __throw_out_of_range() const;
 };