Remove FLAG_COMPILER_GENERATED from user main.

Let the implicit main from the first module named on the command line dominate an explicit
main included in a "use" statement.

Removed the TODO comments related to this regularization of FLAG_COMPILER_GENERATED.
Remove unused labels FLAG_LABEL_BREAK and FLAG_LABEL_CONTINUE.

These labels were unused in the compiler.

TODO: Remove FLAG_INVISIBLE_LABEL as well.  We can just key off of the label symbol.
All labels are invisible labels in scopeResolve.

An assert proved that all labels that are skipped when populating the symbol table in
scopeResolve are invisible labels.  That means that the FLAG_INVISIBLE_LABEL is moot, so I
removed it (and the assert).

Passes all --no-local tests.