[InstrProf] Temporal Profiling

[InstrProf] Temporal Profiling

As described in [0], this extends IRPGO to support //Temporal Profiling//.

When `-pgo-temporal-instrumentation` is used we add the `llvm.instrprof.timestamp()` intrinsic to the entry of functions which in turn gets lowered to a call to the compiler-rt function `INSTR_PROF_PROFILE_SET_TIMESTAMP()`. A new field in the `llvm_prf_cnts` section stores each function's timestamp. Then in `llvm-profdata merge` we convert these function timestamps into a //trace// and add it to the indexed profile.

Since these traces could significantly increase the profile size, we've added `-max-temporal-profile-trace-length` and `-temporal-profile-trace-reservoir-size` to limit the length of a trace and the number of traces in a profile, respectively.

In a future diff we plan to use these traces to construct an optimized function order to reduce the number of page faults during startup.

Special thanks to Julian Mestre for helping with reservoir sampling.

[0] https://discourse.llvm.org/t/rfc-temporal-profiling-extension-for-irpgo/68068

Reviewed By: snehasish

Differential Revision: https://reviews.llvm.org/D147287