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/conf.py b/docs/conf.py
index 70fb71f..6217ead 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -48,9 +48,9 @@
 # built documents.
 #
 # The short X.Y version.
-version = '11.0'
+version = '12.0'
 # The full version, including alpha/beta/rc tags.
-release = '11.0'
+release = '12.0'
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.