Subtype: fix obviously_disjoint for Union Types

[N5N3]

Found in #48935 (comment).

It looks hard to reproduce this error under normal build as obviously_subtype handle Unions correctly.

[N5N3]

@nanosoldier runtests(["TKTDsimulations", "MITgcmTools", "FITSIO", "MLJJLBoost", "Catlab", "QuerySQLite", "EBayes", "OpenTelemetryAPI", "MultiDocumenter", "Elfel", "LocalApproximationValueIteration", "ShellModel", "Arrow", "RelativisticDynamics", "BloqadeODE", "MakieThemes", "UncertainData", "GraphsMatching", "ConstrainedDynamicsVis", "RayTracing", "BioMart", "SymbolicRegression", "DTALib", "RvSpectMLBase", "BufferedStreams", "ReactiveDynamics", "Infernal", "QuantumCumulants", "FMIImport", "GCPDecompositions", "AppliGeneralLedger", "RRTMGP", "FilteredGroupbyMacro", "SubSIt", "HorseML", "EvaluationCF", "Trixi2Vtk", "VoronoiFVM", "NaiveNASflux", "NonconvexMultistart", "MzPlots", "DECAES", "SeaPearlZoo", "AtomGraphs", "AlgebraicAgents", "Rfam", "BioGraph", "BM3DDenoise", "Expect", "OteraEngine", "ContinuousTimePolicyGradients", "ClimaAtmos"], vs = ":release-1.9", configuration = (buildflags = ["LLVM_ASSERTIONS=1", "FORCE_ASSERTIONS=1"],), vs_configuration = (buildflags = ["LLVM_ASSERTIONS=1", "FORCE_ASSERTIONS=1"],))

[nanosoldier]

Your job failed. Consult the server logs for more details (cc @maleadt).

[KristofferC]

I'll merge this and run PkgEval again on the backport branch.

[maleadt]

The same build failure again, resulting in an EAGAIN when building stdlibs. I'll try rebooting the PkgEval server once it finishes the current daily evaluation.