swift234

[SIL] Refactor SILLocation not to derive from PointerUnion3 but to include it as a member

As per Chris's suggestion (review of r6152), further refactored SILLocation not to derive from PointerUnion3 but to include it as a member.

In addition, added some template magic to make sure we don't have to chain dyn_casts, which I suspect will be/is happening a lot with SILLocation:
Ex:
-  if (auto E = Func.dyn_cast<Expr*>()) {
-    if (const FuncExpr *FE = dyn_cast<FuncExpr>(E))
-      return SILLocation(FE->getBody())
+  if (const FuncExpr *FE = Func.getAs<FuncExpr>())
+    return SILLocation(FE->getBody());

Swift SVN r6283