[AST] Make it possible to find what Swift version was used to build a module

For imported modules the version is empty because they don't carry
this information.

lib/AST/Module.cpp

@@ -3935,3 +3935,17 @@ bool IsNonUserModuleRequest::evaluate(Evaluator &evaluator, ModuleDecl *mod) con
   return (!runtimePath.empty() && pathStartsWith(runtimePath, modulePath)) ||
       (!sdkPath.empty() && pathStartsWith(sdkPath, modulePath));
 }
+
+version::Version ModuleDecl::getLanguageVersionBuiltWith() const {
+  for (auto *F : getFiles()) {
+    auto *LD = dyn_cast<LoadedFile>(F);
+    if (!LD)
+      continue;
+
+    auto version = LD->getLanguageVersionBuiltWith();
+    if (!version.empty())
+      return version;
+  }
+
+  return version::Version();
+}