[www-scripts] Clang Attribute Doc Update: ERROR

[root via www-scripts]

UPDATING LLVM SOURCES
HEAD is now at 8c39e70 [mlir][OpFormatGen] Add support for eliding UnitAttr when used to anchor an optional group
>From https://github.com/llvm/llvm-project
 * branch            master     -> FETCH_HEAD
   8c39e70..b5059b7  master     -> origin/master
First, rewinding head to replay your work on top of it...
Fast-forwarded master to b5059b7140232559ed123cb94d4e8f75ca9a44dc.
CMake Error at CMakeLists.txt:3 (cmake_minimum_required):
  CMake 3.13.4 or higher is required.  You are running version 3.7.1


-- Configuring incomplete, errors occurred!