[libc++] Take 2: 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.

Note that this commit had originally been applied in r336369 and then
reverted in r336382 because of unforeseen problems. Both of these problems
have now been fixed.

Reviewers: EricWF, mclow.lists

Subscribers: christof, dexonsmith, erikvanderpoel

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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@336866 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.