[PGO] Exposing PGO's Counter Reset and File Dumping APIs (#76471)

This PR exposes four PGO functions 

- `__llvm_profile_set_filename`
- `__llvm_profile_reset_counters`, 
- `__llvm_profile_dump` 
- `__llvm_orderfile_dump` 

to user programs through the new header `instr_prof_interface.h` under
`compiler-rt/include/profile`. This way, the user can include the header
`profile/instr_prof_interface.h` to introduce these four names to their
programs.

Additionally, this PR defines macro `__LLVM_INSTR_PROFILE_GENERATE` when
the program is compiled with profile generation, and defines macro
`__LLVM_INSTR_PROFILE_USE` when the program is compiled with profile
use. `__LLVM_INSTR_PROFILE_GENERATE` together with
`instr_prof_interface.h` define the PGO functions only when the program
is compiled with profile generation. When profile generation is off,
these PGO functions are defined away and leave no trace in the user's
program.

Background:
https://discourse.llvm.org/t/pgo-are-the-llvm-profile-functions-stable-c-apis-across-llvm-releases/75832