Use MCSymbols for FastISel.

Use MCSymbols for FastISel.

The summary is that it moves the mangling earlier and replaces a few
calls to .addExternalSymbol with addSym.

I originally wanted to replace all the uses of addExternalSymbol with
addSym, but noticed it was a lot of work and doesn't need to be done
all at once.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240395 91177308-0d34-0410-b5e6-96231b3b80d8