StaticMesh Lod Sections use wrong materials after cleaning DDC and regerated from RawMesh

1. Import core_event_marquee_a.FBX to UE4 as a staticmesh。

1. Open the imported staticmesh and import Lod1 from core_event_marquee_a_x1.FBX。

1. Notice Lod1 sections attached with materials correctly.

1. Close editor and clean the DDC of the staticmesh(or change the STATICMESH_DERIVEDDATA_VER and MESHDATAKEY_STATICMESH_DERIVEDDATA_VER in staticmesh.cpp）。

1. Reopen editor and open the staticmesh, the lod1 sections use wrong materials。

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