[FuncSpec] Relax restrictions on candidates for specialisation

Allow a function to be specialised even if it has its address taken or
it's global. For such functions, consider all of the arguments as
overdefined. Don't delete the functions even if all the apparent calls
were redirected to specialised instances.

Reviewed By: labrinea, ChuanqiXu

Differential Revision: https://reviews.llvm.org/D148345