[LTO/WPD] Allow devirtualization to function alias in vtable

Follow on to D144209 to support single implementation devirtualization
for Regular LTO when the vtable holds a function alias.

For now I have prevented other optimizations performed in regular LTO
that need to analyze the contents of the function target when the vtable
holds an alias, as I'm not sure they are always correct to perform in
that case.

Differential Revision: https://reviews.llvm.org/D144270