[AVR] Fix broken bitcast for aliases in non-zero address space

This was triggered by some code in picolibc. The minimal version looks
like this:

    double infinity(void) {
       return 5;
    }

    extern long double infinityl() __attribute__((__alias__("infinity")));

These two declarations have a different type (not because of the 'long
double', which is also 'double' in IR, but because infinityl has
variadic parameters). This led to a crash in the bitcast which assumed
address space 0.

Differential Revision: https://reviews.llvm.org/D138681