Add an "Informative Warnings" Annex

[GoogleCodeExporter]

The C99 Specification includes an "Annex I: (Informative) Common Warnings", 
which is described as:

An implementation may generate warnings in many situations, none of which are 
specified as part of this International Standard. The following are a few of 
the more common situations.

By listing suggested warnings, the UPC Language Specification will aid both 
compiler implementers and users.  The suggested warnings may indirectly clarify 
the meaning of some trickier aspects of the language specification and improve 
the uniformity of diagnostics and checks implemented in various vendor 
compilers.

Original issue reported on code.google.com by gary.funck on 17 Jul 2012 at 6:02