convert conststringvar plugin to shared infrastructre

and move the duplicated hasExternalLinkage function to a common location

Change-Id: I39a1990945666ff7a307b4ddd5e270da64ee1673
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/88592
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

compilerplugins/clang/plugin.cxx

@@ -768,6 +768,31 @@ int derivedFromCount(QualType subclassQt, QualType baseclassQt)
     return derivedFromCount(subclassRecordDecl, baseclassRecordDecl);
 }
 
+// It looks like Clang wrongly implements DR 4
+// (<http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#4>) and treats
+// a variable declared in an 'extern "..." {...}'-style linkage-specification as
+// if it contained the 'extern' specifier:
+bool hasExternalLinkage(VarDecl const * decl) {
+    if (decl->getLinkageAndVisibility().getLinkage() != ExternalLinkage) {
+        return false;
+    }
+    for (auto ctx = decl->getLexicalDeclContext();
+         ctx->getDeclKind() != Decl::TranslationUnit;
+         ctx = ctx->getLexicalParent())
+    {
+        if (auto ls = dyn_cast<LinkageSpecDecl>(ctx)) {
+            if (!ls->hasBraces()) {
+                return true;
+            }
+            if (auto prev = decl->getPreviousDecl()) {
+                return hasExternalLinkage(prev);
+            }
+            return !decl->isInAnonymousNamespace();
+        }
+    }
+    return true;
+}
+
 
 } // namespace