[clang] [llvm] [InstrPGO] Instrument sampling profile based cold function (PR #109837)

[Ellis Hoag via cfe-commits]

ellishg wrote:

Why not use the existing `-pgo-function-entry-coverage` (https://discourse.llvm.org/t/instrprofiling-lightweight-instrumentation/59113/14?u=ellishg) LLVM flag? It takes advantage of the `llvm.instrprof.cover` intrinsic which has less size and runtime overhead than `llvm.instrprof.increment`. It also supports IRPGO and CSIRPGO while it seems that this PR requires a sample PGO input.

https://github.com/llvm/llvm-project/pull/109837