Improve the invalidation situation somewhat when someone defines convert to a Symbol or String

[KristofferC]

 inserting convert(::Type{Symbol}, x::JSON.PtrString) @ JSON ~/.julia/packages/JSON/0oqO1/src/lazy.jl:435 invalidated:
                 16: signature Tuple{typeof(convert), Type{Symbol}, Any} triggered MethodInstance for setindex!(::Dict{Symbol, Bool}, ::Any, ::Any) (650 children)
                 17: signature Tuple{typeof(convert), Type{Symbol}, Any} triggered MethodInstance for @NamedTuple{region::UnitRange{Int64}, label::Symbol, value}(::NamedTuple{(:region, :label, :value)}) (763 children)

Handled by:

• avoid convert calls from type unstable Dict in TermInfo constructor
• force inline of setindex on named tuple in some annotated string code to make the :region field be const known, ref @inline on method declaration leads to worse optimization compared to call-site annotation #60365