MC: Change MCAssembler::Symbols to store MCSymbol, NFC

Instead of storing a list of the `MCSymbolData` in use, store the
`MCSymbol`s.  Churning in the direction of removing the back pointer
from `MCSymbolData`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237496 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed