Don't imply that a compilation unit name is a filename

[jskeet]

In #1120, we've acknowledged that #line can introduce a compilation unit name which isn't a valid filename. We believe there are places which imply/assume that a compilation unit name is a valid filename - we should stop doing so.

Assigned to @Nigel-Ecma for some examples.

[jskeet]

Rex has suggested adding "As such, the accepted spelling of a compilation unit name, and its mapping, if any, to a filename is outside the scope of this specification." to 6.1