Fix ubsan error during CombinedUpstream destruction

[yanavlasov]

Commit Message: This change fixes ubsan error in the TcpTunnelingIntegrationTest.BasicUsePost test. This bug only affects ubsan builds as it can detect use of partially destructed objects. In this case the UpstreamRequest::clearRequestEncoder() which is called during destruction needs access to the CombinedUpstream::decoder_filter_callbacks_ member.

Risk Level: Low Testing: Unit Tests Docs Changes: N/A Release Notes: N/A Platform Specific Features: N/A