[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`.
> 
> We do use the `-pgo-function-entry-coverage` in this PR, see [here](https://github.com/llvm/llvm-project/pull/109837/files#diff-bac41c71569f27df21a843bcd74d2e604ed508afbdf141777761dfb545c5d228R666-R667). but furthermore, we skip instrumenting the functions that are covered by sampling PGO profile.

Oh, I missed that and your [earlier comment](https://github.com/llvm/llvm-project/pull/109837#discussion_r1783584037). Makes more sense, thanks!

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