[swift/master-next] Wrapping reference to SwiftASTContext with #ifndef NDEBUG

Merging #1490 to swift/master-next as requested.

Author: ematejska

lldb/unittests/Symbol/TestSwiftASTContext.cpp

@@ -61,7 +61,9 @@ struct TestSwiftASTContext : public testing::Test {
 };
 
 struct SwiftASTContextTester : public SwiftASTContext {
-  SwiftASTContextTester() : SwiftASTContext() {}
+  #ifndef NDEBUG
+    SwiftASTContextTester() : SwiftASTContext() {}
+  #endif
 
   static std::string GetResourceDir(llvm::StringRef platform_sdk_path,
                                     std::string swift_dir,