Use the new ARMConstantPoolSymbol class to handle external symbols.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140939 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/ARM/ARMAsmPrinter.cpp b/lib/Target/ARM/ARMAsmPrinter.cpp
index 2f1b88e..f4389db 100644
--- a/lib/Target/ARM/ARMAsmPrinter.cpp
+++ b/lib/Target/ARM/ARMAsmPrinter.cpp
@@ -860,7 +860,8 @@
     MCSym = MBB->getSymbol();
   } else {
     assert(ACPV->isExtSymbol() && "unrecognized constant pool value");
-    MCSym = GetExternalSymbolSymbol(ACPV->getSymbol());
+    const char *Sym = cast<ARMConstantPoolSymbol>(ACPV)->getSymbol();
+    MCSym = GetExternalSymbolSymbol(Sym);
   }
 
   // Create an MCSymbol for the reference.