[MemoryBuiltins] Default to trivial mapper in getAllocSize() (NFC)

Default getAllocSize() to use the trivial mapper. Also switch
from using std::function to function_ref.

Furthermore, update the doc comment to point out a subtle difference
between getAllocSize() and getObjectSize(): The latter may also
return something for calls that return their argument (via "returned"
attribute or special intrinsics like invariant groups).