Module::isModule isn't LDC-specific.
Implement previously inoperative pragma(LDC_no_moduleinfo).

GitHub: Fixes #571.
Do not emit ModuleInfo for the implicit __entrypoint module.

This is in line with what DMD does, if of course with a
different implementation.
Merge pull request #671 from klickverbot/no-moduleinfo

Implement previously inoperative pragma(LDC_no_moduleinfo).