[lldb][Fortran] Add TypeSystemFortran files (#218016)
This PR adds the foundational TypeSystemFortran class. To satisfy the
compiler, it implements all required pure virtual functions from the
base TypeSystem class as stubs (returning nullptr or 0).
Changes:
- Adds the TypeSystemFortran class.
- Provides stub implementations for all inherited pure virtual
functions.
Part of the Add Fortran support to LLDB GSoC 2026 project.
Relates to #109119.
