[sil] Add the ability to grab an ASTNode out of a SILLocation.

Previously if one wanted to get an ASTNode, one needed to use the
getAsASTNode<T>() type. If one just wants to get out the type and use it in a
generic way using ASTNode there wasn't any way to do this... so I did it. We
actually have to do the marshalling here since ASTNodeTy and ASTNode have
different layouts despite them both being PointerUnions. So one can't just cast
in between them =---(.

Author: gottesmm

include/swift/SIL/SILLocation.h

@@ -14,6 +14,7 @@
 #define SWIFT_SIL_LOCATION_H
 
 #include "llvm/ADT/PointerUnion.h"
+#include "swift/AST/ASTNode.h"
 #include "swift/Basic/SourceLoc.h"
 #include "swift/SIL/SILAllocated.h"
 #include "swift/AST/TypeAlignments.h"
@@ -414,6 +415,26 @@ class SILLocation {
     return castNodeTo<T>(getPrimaryASTNode());
   }
 
+  /// If this SILLocation contains an ASTNode, return that node.
+  ASTNode getASTNode() const {
+    if (!isASTNode())
+      return ASTNode();
+    // ASTNode is a superset of PrimaryASTNode so we can just cast it, once we
+    // remove the bit stolen by ASTNodeTy from the underlying ASTNode pointer.
+    auto primaryNode = getPrimaryASTNode().getPointer();
+
+    if (auto *stmt = primaryNode.dyn_cast<Stmt *>())
+      return {stmt};
+    if (auto *expr = primaryNode.dyn_cast<Expr *>())
+      return {expr};
+    if (auto *decl = primaryNode.dyn_cast<Decl *>())
+      return {decl};
+    if (auto *pattern = primaryNode.dyn_cast<Pattern *>())
+      return {pattern};
+
+    return ASTNode();
+  }
+
   /// Return the location as a DeclContext or null.
   DeclContext *getAsDeclContext() const;