[sancov] Don't instrument calls to bitcast funcs: they're not indirect.

Currently, when instrumenting indirect calls, this uses
CallBase::getCalledFunction to determine whether a given callsite is
eligible.

However, that returns null if:
  this is an indirect function invocation or the function signature
  does not match the call signature.

So, we end up instrumenting direct calls where the callee is a bitcast
ConstantExpr, even though we presumably don't need to.

Use isIndirectCall to ignore those funky direct calls.

Differential Revision: https://reviews.llvm.org/D119594