MC: Change MCAssembler::Symbols to a vector

Instead of an intrusive double-linked linked list, use a
`std::vector<>`.  This saves a pointer per symbol and simplifies
`MCSymbolData`.  Otherwise, no functionality change here.

While I measured a memory drop from around 1047MB to 1040MB (0.6%) --
and this is a decent cleanup in its own right -- it's primarily a
preparation patch for merging `MCSymbol` and `MCSymbolData`.  I'll post
an updated patch for that to the list in a moment.

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

llvm-svn: 237487