Stop exposing the storage for `MCAssembler::Symbols`, and have
`MCAssembler` add symbols directly to its list instead of using a hook
in `MCSymbolData`. This opens up room for a follow-up commit to switch
from a linked list to a vector.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237486 91177308-0d34-0410-b5e6-96231b3b80d8