gf/codegen: prepare cache and codegen for ConstType in specTypes
Update jl_isa_compileable_sig to accept ConstType elements in specTypes
tuples. Update concretesig_equal and simplett construction to handle
ConstType by extracting the wrapped type via the T field.
In codegen, update is_uniquerep_Type to recognize ConstType as a
uniquely-representable type, and add get_Type_param helper to extract
the type parameter from either Type{T} or ConstType{T}. Update all
ghosting paths to use get_Type_param instead of jl_tparam0.
The actual conversion from Type{T} to ConstType{T} in
jl_compilation_sig is left disabled (as a TODO) until the tuple type
infrastructure is extended to support ConstType elements in specTypes
tuples.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
