[move-function] Implement SILFunction::isDefer().

Returns true if this SILFunction must be a defer.

NOTE: This may return false for defer statements that have been
deserialized without a DeclContext. This means that this is guaranteed to
be correct for SILFunctions in Raw SIL that were not deserialized as
canonical. Thus one can use it for diagnostics.

Author: gottesmm

include/swift/SIL/SILFunction.h

@@ -1056,6 +1056,19 @@ class SILFunction
   /// generic.
   SubstitutionMap getForwardingSubstitutionMap();
 
+  /// Returns true if this SILFunction must be a defer statement.
+  ///
+  /// NOTE: This may return false for defer statements that have been
+  /// deserialized without a DeclContext. This means that this is guaranteed to
+  /// be correct for SILFunctions in Raw SIL that were not deserialized as
+  /// canonical. Thus one can use it for diagnostics.
+  bool isDefer() const {
+    if (auto *dc = getDeclContext())
+      if (auto *decl = dyn_cast_or_null<FuncDecl>(dc->getAsDecl()))
+        return decl->isDeferBody();
+    return false;
+  }
+
   //===--------------------------------------------------------------------===//
   // Block List Access
   //===--------------------------------------------------------------------===//