[libcxx] Remove dynarray

Summary:
std::dynarray had been proposed for C++14, but it was pulled out from C++14
and there are no plans to standardize it anymore.

Reviewers: mclow.lists, EricWF

Subscribers: mgorny, christof, jkorous, dexonsmith, arphaman, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54801

llvm-svn: 347783
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 9a494eacba0e037a213cf18216eb4afbf37410d8
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 4cf5182..b7ad2e2 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -48,3 +48,7 @@
   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.
+- Starting with LLVM 8.0.0, std::dynarray has been removed from the library.
+  std::dynarray was a feature proposed for C++14 that was pulled from the
+  Standard at the last minute and was never standardized. Since there are no
+  plans to standardize this facility it is being removed.