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.

.clang-format

@@ -21,6 +21,7 @@ AlwaysBreakTemplateDeclarations: true
 BinPackArguments: false
 BinPackParameters: false
 BraceWrapping:
+  AfterCaseLabel:  true
   AfterClass:      true
   AfterControlStatement: true
   AfterEnum:       true