[ctx_profile] Profile reader and writer (#91859)

Utility converting a profile coming from `compiler_rt` to bitstream, and
a reader.

`PGOCtxProfileWriter::write` would be used as the `Writer` parameter for
`__llvm_ctx_profile_fetch` API. This is expected to happen in user code,
for example in the RPC hanler tasked with collecting a profile, and
would look like this:

```
// set up an output stream "Out", which could contain other stuff
{
  // constructing the Writer will start the section, in Out, containing
  // the collected contextual profiles.
  PGOCtxProfWriter Writer(Out);
  __llvm_ctx_profile_fetch(&Writer, +[](void* W, const ContextNode &N) {
    reinterpret_cast<PGOCtxProfWriter*>(W)->write(N);
  });
  // Writer going out of scope will finish up the section.
}
```

The reader produces a data structure suitable for maintenance during IPO
transformations.