Drop SrcStructTypesSet. It is redundant.

At the only point in the code it is used, we haven't added any of the src types
to DstStructTypesSet yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223057 91177308-0d34-0410-b5e6-96231b3b80d8