[TypeChecker/Constness] Diagnostics: Walk into multi-statement closures when inference is enabled

When multi-statement closure inference is enabled it's body is
type-checked together with enclosing context, so they could be
walked directly just like single-expressions ones.

Author: xedin

lib/Sema/ConstantnessSemaDiagnostics.cpp

@@ -359,10 +359,13 @@ void swift::diagnoseConstantArgumentRequirement(
     }
 
     std::pair<bool, Expr *> walkToClosureExprPre(ClosureExpr *closure) {
-      if (closure->hasSingleExpressionBody()) {
-        // Single expression closure bodies are not visited directly
-        // by the ASTVisitor, so we must descend into the body manually
-        // and set the DeclContext to that of the closure.
+      auto &ctx = DC->getASTContext();
+
+      if (closure->hasSingleExpressionBody() ||
+          ctx.TypeCheckerOpts.EnableMultiStatementClosureInference) {
+        // Closure bodies are not visited directly by the ASTVisitor,
+        // so we must descend into the body manuall and set the
+        // DeclContext to that of the closure.
         DC = closure;
         return {true, closure};
       }