[flang] Add explicit '= default' for move constructor.

Original-commit: flang-compiler/f18@7007a488ba
Reviewed-on: https://github.com/flang-compiler/f18/pull/186
Tree-same-pre-rewrite: false

flang/lib/semantics/resolve-labels.cc

@@ -216,6 +216,7 @@ struct UnitAnalysis {
 
 class ParseTreeAnalyzer {
 public:
+  ParseTreeAnalyzer(ParseTreeAnalyzer &&that) = default;
   ParseTreeAnalyzer(parser::Messages &errorHandler)
     : errorHandler_{errorHandler} {}