[libc++] Use exclude_from_explicit_instantiation instead of always_inline

Summary:
This commit adopts the exclude_from_explicit_instantiation attribute discussed
at [1] and reviewed in [2] in libc++ to supplant the use of __always_inline__
for visibility purposes.

This change means that users wanting to link together translation units built
with different versions of libc++'s headers into the same final linked image
MUST define the _LIBCPP_HIDE_FROM_ABI_PER_TU macro to 1 when building those
TUs. Doing otherwise will lead to ODR violations and ABI issues.

[1]: http://lists.llvm.org/pipermail/cfe-dev/2018-August/059024.html
[2]: https://reviews.llvm.org/D51789

Reviewers: rsmith, EricWF

Subscribers: dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D52405

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@345516 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.rst

@@ -41,3 +41,10 @@ New Features
 API Changes
 -----------
 - Building libc++ for Mac OSX 10.6 is not supported anymore.
+- Starting with LLVM 8.0.0, users that wish to link together translation units
+  built with different versions of libc++'s headers into the same final linked
+  image MUST define the _LIBCPP_HIDE_FROM_ABI_PER_TU macro to 1 when building
+  those translation units. Not defining _LIBCPP_HIDE_FROM_ABI_PER_TU to 1 and
+  linking translation units built with different versions of libc++'s headers
+  together may lead to ODR violations and ABI issues. On the flipside, code
+  size improvements should be expected for everyone not defining the macro.