Index: lldb/include/lldb/Symbol/ClangExternalASTSourceCommon.h
===================================================================
--- lldb/include/lldb/Symbol/ClangExternalASTSourceCommon.h
+++ lldb/include/lldb/Symbol/ClangExternalASTSourceCommon.h
@@ -122,14 +122,18 @@
 };
 
 class ClangExternalASTSourceCommon : public clang::ExternalASTSource {
+  static char ID;
+
 public:
-  ClangExternalASTSourceCommon();
+  ClangExternalASTSourceCommon() = default;
   ~ClangExternalASTSourceCommon() override;
 
   ClangASTMetadata *GetMetadata(const void *object);
   void SetMetadata(const void *object, ClangASTMetadata &metadata);
 
-  static ClangExternalASTSourceCommon *Lookup(clang::ExternalASTSource *source);
+  // LLVM-style RTTI.
+  bool isA(const void *ClassID) const override { return ClassID == &ID; }
+  static bool classof(const ExternalASTSource *S) { return S->isA(&ID); }
 
 private:
   typedef llvm::DenseMap<const void *, ClangASTMetadata> MetadataMap;
Index: lldb/source/Symbol/ClangASTContext.cpp
===================================================================
--- lldb/source/Symbol/ClangASTContext.cpp
+++ lldb/source/Symbol/ClangASTContext.cpp
@@ -2407,22 +2407,18 @@
 
 void ClangASTContext::SetMetadata(clang::ASTContext *ast, const void *object,
                                   ClangASTMetadata &metadata) {
-  ClangExternalASTSourceCommon *external_source =
-      ClangExternalASTSourceCommon::Lookup(ast->getExternalSource());
-
-  if (external_source)
-    external_source->SetMetadata(object, metadata);
+  if (auto *A = llvm::dyn_cast_or_null<ClangExternalASTSourceCommon>(
+          ast->getExternalSource()))
+    A->SetMetadata(object, metadata);
 }
 
 ClangASTMetadata *ClangASTContext::GetMetadata(clang::ASTContext *ast,
                                                const void *object) {
-  ClangExternalASTSourceCommon *external_source =
-      ClangExternalASTSourceCommon::Lookup(ast->getExternalSource());
 
-  if (external_source)
-    return external_source->GetMetadata(object);
-  else
-    return nullptr;
+  if (auto *A = llvm::dyn_cast_or_null<ClangExternalASTSourceCommon>(
+          ast->getExternalSource()))
+    return A->GetMetadata(object);
+  return nullptr;
 }
 
 bool ClangASTContext::SetTagTypeKind(clang::QualType tag_qual_type,
Index: lldb/source/Symbol/ClangExternalASTSourceCommon.cpp
===================================================================
--- lldb/source/Symbol/ClangExternalASTSourceCommon.cpp
+++ lldb/source/Symbol/ClangExternalASTSourceCommon.cpp
@@ -13,43 +13,9 @@
 
 using namespace lldb_private;
 
-typedef llvm::DenseMap<clang::ExternalASTSource *,
-                       ClangExternalASTSourceCommon *>
-    ASTSourceMap;
+char ClangExternalASTSourceCommon::ID;
 
-static ASTSourceMap &GetSourceMap(std::unique_lock<std::mutex> &guard) {
-  // Intentionally leaked to avoid problems with global destructors.
-  static ASTSourceMap *s_source_map = new ASTSourceMap;
-  static std::mutex s_mutex;
-  std::unique_lock<std::mutex> locked_guard(s_mutex);
-  guard.swap(locked_guard);
-  return *s_source_map;
-}
-
-ClangExternalASTSourceCommon *
-ClangExternalASTSourceCommon::Lookup(clang::ExternalASTSource *source) {
-  std::unique_lock<std::mutex> guard;
-  ASTSourceMap &source_map = GetSourceMap(guard);
-
-  ASTSourceMap::iterator iter = source_map.find(source);
-
-  if (iter != source_map.end()) {
-    return iter->second;
-  } else {
-    return nullptr;
-  }
-}
-
-ClangExternalASTSourceCommon::ClangExternalASTSourceCommon()
-    : clang::ExternalASTSource() {
-  std::unique_lock<std::mutex> guard;
-  GetSourceMap(guard)[this] = this;
-}
-
-ClangExternalASTSourceCommon::~ClangExternalASTSourceCommon() {
-  std::unique_lock<std::mutex> guard;
-  GetSourceMap(guard).erase(this);
-}
+ClangExternalASTSourceCommon::~ClangExternalASTSourceCommon() {}
 
 ClangASTMetadata *
 ClangExternalASTSourceCommon::GetMetadata(const void *object) {