Add Profile-Guided Optimization (PGO) support to the Flang compiler

[zamazan4ik]

Similar to Clang, we need to support Profile-Guided Optimization (PGO) in Flang too. Use cases for having this optimization are completely the same as we have for Clang right now - have an ability to optimize better Fortran code based on the runtime information.

I propose adding the following PGO types for Flang:

• IR PGO (-fprofile-generate/-fprofile-use options in Clang)
• CS PGO (fcs-profile-generate in Clang). This option would be a good addition to IR PGO
• Sampling PGO (AutoFDO + -fprofile-sample-use option in Clang). Having sampling PGO support is important if we want to collect PGO profiles directly from a production environment.

I think there is no need to implement FE PGO (-fprofile-instr-generate/-fprofile-instr-use options in Clang) in Flang since IR PGO is a better option according to the issue.

As a reference, the Clang PGO docs can be used.