[clang][ssaf] Add FormatInfo sub-registry and tests [2/2] (#179517)
Add `FormatInfoEntry` template to support per-analysis-type
serialization within a `SerializationFormat`.
This allows to implement different formats for the different analyses in
a decoupled way.
For testing, this patch also implements the MockSerializationFormat
demonstrating the FormatInfo sub-registry pattern.
Assisted-by: claude
Depends on #179516
rdar://169192127