Update to use clangformat9

With clangformat9 the AfterCaseLabel is introduced.
And this defaults to false, but our code is formatted
implicitly with this set to true.

PRs to Verona and Snmalloc are being formatted with clangformat9,
and this is causing complexity. Let's move forward to clangformat9
in CI.

CMakeLists.txt

@@ -35,15 +35,12 @@ endmacro()
 
 macro(clangformat_targets)
   # The clang-format tool is installed under a variety of different names.  Try
-  # to find a sensible one.  Only look for versions 6 to 8 explicitly - we don't
+  # to find a sensible one.  Only look for versions 9 explicitly - we don't
   # know whether our clang-format file will work with newer versions of the
-  # tool
+  # tool.  It does not work with older versions as AfterCaseLabel is not supported
+  # in earlier versions.
   find_program(CLANG_FORMAT NAMES
-    clang-format-8
-    clang-format-7.0
-    clang-format-6.0
-    clang-format70
-    clang-format60
+    clang-format-9
     clang-format)
 
   # If we've found a clang-format tool, generate a target for it, otherwise emit