[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

docs/DesignDocs/VisibilityMacros.rst

@@ -41,10 +41,10 @@ Visibility Macros
   library and has an empty definition otherwise.
 
 **_LIBCPP_INLINE_VISIBILITY**
-  Mark a function as hidden and force inlining whenever possible.
-
-**_LIBCPP_ALWAYS_INLINE**
-  A synonym for `_LIBCPP_INLINE_VISIBILITY`
+  Mark a function as not being part of the ABI of any final linked image that
+  uses it, and also as being internal to each TU that uses that function. In
+  other words, the address of a function marked with this attribute is not
+  guaranteed to be the same across translation units.
 
 **_LIBCPP_TYPE_VIS**
   Mark a type's typeinfo, vtable and members as having default visibility.