Lexical redeclarations in subs should throw exceptions

[chromatic]

This code should throw an exception:

.sub 'main' :main
    .lex 'foo', $P0
    .lex 'bar', $P1
    .lex 'foo', $P2
.end

I'll add a TODO test for it in t/compilers/imcc/syn/clash.t when I get this ticket's number.

Please note that adding this exception to IMCC is fairly intractable without rewriting most of IMCC's symbol handling. I believe our best chance to make this work is with pirc.

[kjs]

Implemented in r42079 in PIRC. Not sure whether it's "wontfix" for IMCC (?).

[plobsing]

Fixed in r46449.

It detects the multiple definitions at the end of the sub, so the line number is slightly misleading; but it does tell you what you did and in which sub you did it, so I'm calling it "Good Enough"(TM).