The idea is to separate the notion of const from 'this variable can always be
replaced with its initializer' in the frontend. To do that, I introduced
Declaration::isSameAsInitializer, which is overridden in VarDeclaration to
return false for constants that have a struct literal initializer.
So
{{{
const S s = S(5);
void foo() { auto ps = &s; }
// is no longer replaced by
void foo() { auto ps = &(S(5)); }
}}}
To make taking the address of a struct constant with a struct-initializer
outside of function scope possible, I made sure that AddrExp::optimize doesn't
try to run the argument's optimization with WANTinterpret - that'd again
replace the constant with a struct literal temporary.
