Support the capture-thrift-isset feature with TreeBuilder-v2. Fairly minor
changes here except the type of the Enum in a template parameter now matters.
We follow the previous behaviour of capturing a value for each field in a
struct that has an `isset_bitset`. This value is a VarInt captured before the
C++ contents of the member. It has 3 values: 0 (not set), 1 (set), and 2
(unavailable). These are handled by the processor and represented in the output
as `false`, `true`, and `std::nullopt_t` respectively.
Changes:
- Add a simple Thrift isset processor before any fields that have Thrift isset.
- Store the fully qualified names of enum types in DrgnParser - it already
worked out this information anyway for naming the values and this is
consistent with classes.
- Forward all enum template parameters under their input name under the
assumption that they will all be policy type things like `IssetBitsetOption`.
This could turn out to be wrong.
Test plan:
- CI (doesn't test thrift changes but covers other regressions)
- Updated Thrift enum tests for new format.
- `FILTER='OilIntegration.*' make test` - Thrift tests failed before, succeed
after.
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