[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.

(cherry picked from commit fe6e3f9)

Author: gottesmm

include/swift/SIL/SILFunction.h

@@ -1029,6 +1029,19 @@ 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
   //===--------------------------------------------------------------------===//