Don't use InMemoryStruct<macho::SymtabLoadCommand>.

This also required not using the RegisterStringTable API, which is also a
good thing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178947 91177308-0d34-0410-b5e6-96231b3b80d8
Remove last use of InMemoryStruct from MachOObjectFile.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178948 91177308-0d34-0410-b5e6-96231b3b80d8