Bump --enable-compiler-plugins to Clang 3.8.0

<https://lists.freedesktop.org/archives/libreoffice/2017-December/079107.html>
"Clang baseline bump"

Change-Id: I18fca8794ea34118fc6308458064d0c28cf5caf7
Reviewed-on: https://gerrit.libreoffice.org/46557
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

compilerplugins/clang/getimplementationname.cxx

@@ -13,7 +13,6 @@
 // only compile this on clang 3.7 or higher, which is known to work
 // there were problems on clang 3.5 at least
 #include "config_clang.h"
-#if CLANG_VERSION >= 30700
 #include <cassert>
 #include <stdlib.h>
 #include <string>
@@ -312,6 +311,5 @@ loplugin::Plugin::Registration<GetImplementationName> X(
     "getimplementationname", false);
 }
 #endif
-#endif
 
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