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.

llvm-svn: 240395
diff --git a/llvm/lib/Target/AArch64/AArch64MCInstLower.cpp b/llvm/lib/Target/AArch64/AArch64MCInstLower.cpp
index e55ae99..580427a 100644
--- a/llvm/lib/Target/AArch64/AArch64MCInstLower.cpp
+++ b/llvm/lib/Target/AArch64/AArch64MCInstLower.cpp
@@ -187,6 +187,9 @@
   case MachineOperand::MO_ExternalSymbol:
     MCOp = LowerSymbolOperand(MO, GetExternalSymbolSymbol(MO));
     break;
+  case MachineOperand::MO_MCSymbol:
+    MCOp = LowerSymbolOperand(MO, MO.getMCSymbol());
+    break;
   case MachineOperand::MO_JumpTableIndex:
     MCOp = LowerSymbolOperand(MO, Printer.GetJTISymbol(MO.getIndex()));
     break;