[clangd] Fix a use-after-free

[kadircet]

Summary

Clangd was storing reference to a possibly-dead string in compiled
config. This patch fixes the issue by copying suppression strings from
fragments into compiled Config.

Fixes https://github.com/clangd/clangd/issues/724.

[njames93]

clang-tools-extra/clangd/ConfigCompile.cpp

385–386

While were here, worth while to move Normalized into the lambda.

clang-tools-extra/clangd/ConfigCompile.cpp

Out.Apply.push_back([Spec(std::move(Spec))](const Params &P, Config &C) {
       // Disable background indexing for the files under the mountpoint.
       // Note that this will overwrite statements in any previous fragments
       // (including the current one).
       C.Index.Background = Config::BackgroundPolicy::Skip;
     });
   }
 
   void compile(Fragment::DiagnosticsBlock &&F) {
-    std::vector<llvm::StringRef> Normalized;
+    std::vector<std::string> Normalized;
     for (const auto &Suppressed : F.Suppress) {
       if (*Suppressed == "*") {
         Out.Apply.push_back([&](const Params &, Config &C) {
           C.Diagnostics.SuppressAll = true;
           C.Diagnostics.Suppress.clear();
         });
         return;
       }
-      Normalized.push_back(normalizeSuppressedCode(*Suppressed));
+      Normalized.push_back(normalizeSuppressedCode(*Suppressed).str());
     }
     if (!Normalized.empty())
-      Out.Apply.push_back([Normalized](const Params &, Config &C) {
+      Out.Apply.push_back(
+          [Normalized(std::move(Normalized))](const Params &, Config &C) {
             if (C.Diagnostics.SuppressAll)
               return;
             for (llvm::StringRef N : Normalized)
               C.Diagnostics.Suppress.insert(N);
           });
 
     compile(std::move(F.ClangTidy));
   }
 
   void compile(Fragment::StyleBlock &&F) {
     if (!F.FullyQualifiedNamespaces.empty()) {
       std::vector<std::string> FullyQualifiedNamespaces;
       for (auto &N : F.FullyQualifiedNamespaces) {