MC: Make MCSymbolData::Symbol private

Make the back-pointer from `MCSymbolData` to `MCSymbol` private,
preparing to remove the back pointer entirely.  I've already updated all
the users, although for now it's still used to indicate whether
`MCSymbol::Data` has been initialized.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237868 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed