create clang plugin to warn about C-style casts

We don't like C-style casts in our nice C++ code

Change-Id: I94e7ec90de9275cd6e20c4146d4f3a74bed93c9d
Reviewed-on: https://gerrit.libreoffice.org/10367
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>