CHROMIUM: kbuild: clang: Disable the 'duplicate-decl-specifier' warning

clang generates plenty of these warnings in different parts of the code.
They are mostly caused by container_of() and other macros which declare
a "const <type> *" variable for their internal use which triggers a
"duplicate 'const' specifier" warning if the <type> is already const
qualified.

Change-Id: I85ffb201003d3a04fe8b8ff94478344250d2db68
Wording-mostly-from: Michael Davidson <md@google.com>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Greg Hackmann <ghackmann@google.com>