Bump the trunk major version to 12

and clear the release notes.

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 7ab7b979d29e1e43701cf690f5cf1903740f50e3
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 1db7915..5157685 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -1,5 +1,5 @@
 =========================================
-Libc++ 11.0.0 (In-Progress) Release Notes
+Libc++ 12.0.0 (In-Progress) Release Notes
 =========================================
 
 .. contents::
@@ -10,7 +10,7 @@
 
 .. warning::
 
-   These are in-progress notes for the upcoming libc++ 11 release.
+   These are in-progress notes for the upcoming libc++ 12 release.
    Release notes for previous releases can be found on
    `the Download Page <https://releases.llvm.org/download.html>`_.
 
@@ -18,7 +18,7 @@
 ============
 
 This document contains the release notes for the libc++ C++ Standard Library,
-part of the LLVM Compiler Infrastructure, release 11.0.0. Here we describe the
+part of the LLVM Compiler Infrastructure, release 12.0.0. Here we describe the
 status of libc++ in some detail, including major improvements from the previous
 release and new feature work. For the general LLVM release notes, see `the LLVM
 documentation <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM releases may
@@ -32,13 +32,13 @@
 the current one. To see the release notes for a specific release, please
 see the `releases page <https://llvm.org/releases/>`_.
 
-What's New in Libc++ 11.0.0?
+What's New in Libc++ 12.0.0?
 ============================
 
 New Features
 ------------
 
-- ``<numbers>``
+- ...
 
 API Changes
 -----------