This is a follow-up from PR #122545, which enabled converting yaml to contextual profiles.
This change uses the lower level yaml APIs because:
- the mapping APIs `llvm::yaml` offers don't work with `const` values, because they (the APIs) want to enable both serialization and deserialization
- building a helper data structure would be an alternative, but it'd be either memory-consuming or overly-complex design, given the recursive nature of the contextual profiles.
- the set used for targets under a callsite is simpler to use if iterators
are stable (it gets manipulated during updates)
- the set used to fetch the transitive closure of GUIDs under a node can
be left as a choice to the user.
This requires output-ing a "Magic" 4-byte header. We also emit a block info block, to describe our blocks and records. The output of `llvm-bcanalyzer` would look like:
```
<BLOCKINFO_BLOCK/>
<Metadata NumWords=17 BlockCodeSize=2>
<Version op0=1/>
<Context NumWords=13 BlockCodeSize=2>
<GUID op0=2/>
<Counters op0=1 op1=2 op2=3/>
```
Instead of having `Unknown` for block and record IDs.
This reverts commit 03c7458a3603396d2d0e1dee43399d3d1664a264.
One of the problems was addressed in #92208
The other problem: needed to add `BitstreamReader` to the list of
link deps of `LLVMProfileData`
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.
