This appears to have been accidentally added in
d8cc75946b as part of the changes to
support C++11 rvalue references, presumably to match the existing
correct rule for unary &.
There is a non-standard GCC extension for taking the address of a label
using unary &&:
https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html
However this extension only works in an expression in a function body,
and SWIG doesn't parse the contents of function bodies so this grammar
rule isn't even providing any useful support for this extension.