RFR 8203436: javac should fail early when emitting illegal signature attributes

[Maurizio Cimadamore]

Hi,
this patch makes javac a bit stricter when it comes to generating 
bytecode that has illegal signature attributes (which can happen in case 
of non-denotable types). Instead of allowing compilation and then fail 
at verify time, this patch issues an error message.

I've refactored another generation-related diagnostic to share the same 
'header' - an example of the diagnostics involved is included in the webrev.

http://cr.openjdk.java.net/~mcimadamore/8203436/

Cheers
Maurizio