[flang][NFC] Add a no-arg constructor for `Verbatim`.

In C++20, types that declare or delete any constructors are no longer aggregates, breaking compilation of many existing uses of aggregate initialization.

Although `Verbatim` declares itself to not have a no-arg default constructor, this is circumvented in `basic-parsers.h` which returns a `RESULT{}` a.k.a. `Verbatim{}`. Adding the no-arg constructor while still deleting the copy/assignment constructors maintains the current state and also supports eventually building this in c++20 mode.

Fix suggested in https://discourse.llvm.org/t/build-failure-when-attempting-to-build-flang-with-c-20/66953.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D139228