[clang] [CMake][PGO] Use check targets to generate profdata for PGO builds (PR #77347)

[Petr Hosek via cfe-commits]

petrhosek wrote:

I'd also prefer to make this configurable, we're using our own corpus which in my experiments both produces better results and takes less time than `check-llvm` and `check-clang`.

We should also consider updating the documentation since I don't think that `check-llvm` and `check-clang` is what we should be recommending; LLVM and Clang tests have different goal, that is to provide maximum coverage which often means exercising various corner cases and error conditions, but these are not helpful when collecting profiles, in fact they can be harmful.

I think that [LLVM test-suite](https://github.com/llvm/llvm-test-suite/) is actually a better fit since more representative of real code. In our  training corpus, I for example use [CppPerformanceBenchmarks](https://gitlab.com/chriscox/CppPerformanceBenchmarks) (an older version is also [included in LLVM test suite](https://github.com/llvm/llvm-test-suite/tree/9ca97f5027150f7e507e5ab4c56f38a29fb3c696/SingleSource/Benchmarks/Adobe-C%2B%2B)).

https://github.com/llvm/llvm-project/pull/77347