Improve Clang 3.2 hasCLanguageLinkageType workaround

Change-Id: If5c104adc05aa362d15cedf6f8953bda478897bc

compilerplugins/clang/implicitboolconversion.cxx

@@ -85,7 +85,7 @@ bool hasCLanguageLinkageType(FunctionDecl const * decl) {
         return true;
     }
 #else
-    if (decl->getDeclContext()->isExternCContext()) {
+    if (decl->getCanonicalDecl()->getDeclContext()->isExternCContext()) {
         return true;
     }
 #endif