Either deprecate or strengthen upc_addrfield()

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Steve's comment, split off from issue 106:

Consider the following code:

shared int *S1;
shared [] int *S2;

S2 = S1;

if ( upc_addrfield(S1) == upc_addrfield(S2) ) {
    printf("Match\n");
}

With the proposed change (as well as UPC 1.2 apparently), it is undefined 
whether or not anything is printed.  We don't require anywhere that 
pointers-to-shared with different types that point to the same object produce 
the same result when passed to upc_addrfield(), merely that such pointers shall 
compare equal.  Since I don't think this is intended, we probably need a 
stronger statement somewhere to make upc_addrfield() useful.

Original issue reported on code.google.com by danbonachea on 27 Feb 2013 at 11:06