[FS-AFDO] Assign discriminators to pseudo probes

[FS-AFDO] Assign discriminators to pseudo probes

This is the first change for FS-AFDO integration with CSSPGO. There are more patches coming.

With pseudo probes, we do not assign FS discriminators to any other instructions since we will be using only probes for profile correlation.

Also call instructions are excluded since their dwarf discriminators are used for other purposes, i.e, storing probe ids. Since they are not getting a FS discriminator, they will also be excluded from MIR profile loading. The corresponding changes will be in the subsequent patches.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D147286