[clang] [WIP] MicrosoftCXXABI: Fix missing forward declaration in ExternalASTSource.h (#53486)

Author: glebov-andrey

clang/include/clang/AST/ExternalASTSource.h

@@ -40,6 +40,7 @@ class ASTConsumer;
 class ASTContext;
 class ASTSourceDescriptor;
 class CXXBaseSpecifier;
+class CXXConstructorDecl;
 class CXXCtorInitializer;
 class CXXRecordDecl;
 class DeclarationName;