MC: Simplify MCSymbolData initialization and remove MCSymbol pointer

Finally remove the `MCSymbolData::Symbol` pointer.  It was still being
used to track whether `MCSymbolData` had been initialized, but this is
better tracked by the bitfield in `MCSymbol`.

The only caller of `MCSymbolData::initialize()` was `MCAssembler`, which
(other than `Symbol`) passed in all-0 values.  Replace all that
indirection with a default constructor.

The main point is a cleanup (and there's more cleanup to do), but there
are also some small memory savings.  I measured ~989 MB down to ~975 MB,
cutting a little over 1% off the top of `llc`.

(I'm looking at `llc` memory usage on `verify-uselistorder.lto.opt.bc`;
see r236629 for details.)

llvm-svn: 237873