Wrapping reference to SwiftASTContext with #ifndef NDEBUG

Wrapping reference to SwiftASTContext with #ifndef NDEBUG/#endif because it was only defined in that case per: 5fa7d8e.   Without this, running the full lldb testsuite seems to fail on this test.

Author: ematejska

lldb/unittests/Symbol/TestSwiftASTContext.cpp

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