[SampleFDO] Make FSDiscriminator flag part of function parameters

Add a parameter of IsFSDiscriminator to function
getBaseDiscriminatorFromDiscriminator().

This function currently checks the internal flag of
--enable-fs-discriminator. This is not good because we might
change the default value of the internal flag.

Note that we have a default parameter. This is just
because create_afdo_tool has a call-site to it.
I will remove the default parameter in a later patch.

Differential Revision: https://reviews.llvm.org/D104584