Introduce the notion of function addresses in Subzero.

src/IceDefs.h

Index: src/IceDefs.h
diff --git a/src/IceDefs.h b/src/IceDefs.h
index 29ed5d814af847a48b3e28ca311723518abf2307..653d02b73b34eaf466b494db20f53f9a59a2bc85 100644
--- a/src/IceDefs.h
+++ b/src/IceDefs.h
@@ -40,6 +40,7 @@ class Cfg;
 class CfgNode;
 class Constant;
 class GlobalContext;
+class GlobalAddress;
 class Inst;
 class InstPhi;
 class InstTarget;