[BOLT] Add main fragment to function layout

Functions that do not contain any code still have to be emitted. This
occurs on AArch64 where functions can consist only of a constant island.
To support fragment semantics in code emission, this commits adds a
guaranteed main fragment to function layout. This fragment might be
empty, but allows us omit checks whether the function is empty in most
places.

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D130051