[Telemetry] Fix a warning

This patch fixes:

  llvm/include/llvm/Telemetry/Telemetry.h:66:8: error:
  'llvm::telemetry::Config' has virtual functions but non-virtual
  destructor [-Werror,-Wnon-virtual-dtor]

Author: kazutakahirata

llvm/include/llvm/Telemetry/Telemetry.h

@@ -71,6 +71,8 @@ struct Config {
 
   explicit Config() : EnableTelemetry(BuildTimeEnableTelemetry) {}
 
+  virtual ~Config() = default;
+
   // Telemetry can only be enabled if both the runtime and buildtime flag
   // are set.
   explicit Config(bool E) : EnableTelemetry(E && BuildTimeEnableTelemetry) {}