[FS-AFDO][llvm-profgen] Generate profile with FS-AFDO discriminator

In order to support generating profile  with FS discriminator, three kind of changes are done in llvm-profgen:

1) Dissassemble .rodata section to check if FS discriminator var ('"__llvm_fs_discriminator__"') exists and set the corresponding flag in the binary.

2) Change the discriminator decoding in `getBaseDiscriminator` and `getDuplicationFactor`.

3) set true for `FunctionSamples::ProfileIsFS` to enable FS functionality in ProfileData.

Reviewed By: xur, hoy, wenlei

Differential Revision: https://reviews.llvm.org/D113296