[ConstraintSystem] Augment AppliedBuilderTransform to reference transformed body

If result builder AST transform is enabled, let builder transform descriptor
carry tranformed body for ease of access during solution application.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -886,6 +886,10 @@ struct AppliedBuilderTransform {
   /// converted. Opaque types should be unopened.
   Type bodyResultType;
 
+  /// The version of the original body with result builder applied
+  /// as AST transformation.
+  NullablePtr<BraceStmt> transformedBody;
+
   /// An expression whose value has been recorded for later use.
   struct RecordedExpr {
     /// The temporary value that captures the value of the expression, if