make MachineModuleInfoMachO hold non-const MCSymbol*'s instead

make MachineModuleInfoMachO hold non-const MCSymbol*'s instead
of const ones.  non-const ones aren't very useful, because you can't
even, say, emit them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95205 91177308-0d34-0410-b5e6-96231b3b80d8