[ctxprof] Move test serialization to yaml (#122545)

We have a textual representation of contextual profiles for test scenarios, mainly. This patch moves that to YAML instead of JSON. YAML is more su

[ctxprof] Move test serialization to yaml (#122545)

We have a textual representation of contextual profiles for test scenarios, mainly. This patch moves that to YAML instead of JSON. YAML is more succinct and readable (some of the .ll tests should be illustrative). In addition, JSON is parse-able by the YAML reader.

A subsequent patch will address deserialization.

(thanks, @kazutakahirata, for showing me how to use the llvm YAML reader/writer APIs, which I incorrectly thought to be more low-level than the JSON ones!)

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[nfc][ctx_prof] Test comments and more clarity in values.

[ctx_prof] Handle case when no root is in this Module. (#107463)

If none of the functions in this `Module` are roots in the contextual profile, we can't use it and should just return the `{}` case.