Two comments:

1) There surely must be a better name for this? `canOmitFunction`? `skipEmittingFunction`? …

2) I don't know how GDC handles things (@ibuclaw), but LDC recently transitioned to using the same scheme as DMD to determine whether to emit a symbol to the current module or not. This means that this function is also useful for us, and I'd thus like see it being moved out of `glue.c` which is not part of LDC (as is the rest of the glue code).