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/unusedmethodsremove.cxx

@@ -76,7 +76,7 @@ UnusedMethodsRemove::~UnusedMethodsRemove()
 std::string niceName(const CXXMethodDecl* functionDecl)
 {
     std::string s =
-        compat::getReturnType(*functionDecl).getCanonicalType().getAsString()
+        functionDecl->getReturnType().getCanonicalType().getAsString()
         + " " + functionDecl->getParent()->getQualifiedNameAsString()
         + "::" + functionDecl->getNameAsString()
         + "(";