[dmd-internals] Two regression patches

[Don Clugston]

There are currently ten regressions from 2.030 and later. There are
patches for two of them:

4516 Regression(2.040): forward declaration of enum not supported

The patch for
4503  forward reference to aliased template instance
also fixes
3679 Regression(2.031) template forward reference regression
4267 forward reference error when 2-fold aliasing a template instance

I've also made a mitigation patch to change this wrong-code into an
unimportant rejects-valid:
3326 $ in delegate literal causes Access Violation