Cannot handle `file: Nothing` LineNumberNodes

[cstjean]

MacroTools' @q and @qq are macros that behave like quote, but @q strips out all LineInfo and @qq is "Like the quote keyword but replace construction site line numbers with __source__". I was debugging some MacroTools issue and found that this code fails:

using MacroTools: @qq, @q
using JuliaInterpreter

macro inner()
    @qq begin end
end

macro outer()
    @q @inner()
end

foo() = @outer

@interpret foo()

output:

ERROR: LoadError: MethodError: Cannot `convert` an object of type Nothing to an object of type Symbol
The function `convert` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  Symbol(::Any...)
   @ Base strings/basic.jl:229
  convert(::Type{T}, ::T) where T
   @ Base Base.jl:126

Stacktrace:
  [1] setindex!(h::Dict{Symbol, Nothing}, v0::Nothing, key0::Nothing)
    @ Base ./dict.jl:346
  [2] push!(s::Set{Symbol}, x::Nothing)
    @ Base ./set.jl:137
  [3] JuliaInterpreter.FrameCode(scope::Method, src::Core.CodeInfo; generator::Bool, optimize::Bool)
    @ JuliaInterpreter ~/.julia/packages/JuliaInterpreter/nxxiG/src/types.jl:165
  [4] FrameCode
    @ ~/.julia/packages/JuliaInterpreter/nxxiG/src/types.jl:126 [inlined]
  [5] prepare_framecode(method::Method, argtypes::Any; enter_generated::Bool)
    @ JuliaInterpreter ~/.julia/packages/JuliaInterpreter/nxxiG/src/construct.jl:170
  [6] prepare_framecode
    @ ~/.julia/packages/JuliaInterpreter/nxxiG/src/construct.jl:130 [inlined]
  [7] prepare_call(f::Any, allargs::Vector{Any}; enter_generated::Bool)
    @ JuliaInterpreter ~/.julia/packages/JuliaInterpreter/nxxiG/src/construct.jl:250
  [8] prepare_call
    @ ~/.julia/packages/JuliaInterpreter/nxxiG/src/construct.jl:227 [inlined]
  [9] determine_method_for_expr(expr::Expr; enter_generated::Bool)
    @ JuliaInterpreter ~/.julia/packages/JuliaInterpreter/nxxiG/src/construct.jl:586
 [10] determine_method_for_expr
    @ ~/.julia/packages/JuliaInterpreter/nxxiG/src/construct.jl:577 [inlined]
 [11] enter_call_expr(expr::Expr; enter_generated::Bool)
    @ JuliaInterpreter ~/.julia/packages/JuliaInterpreter/nxxiG/src/construct.jl:625
 [12] enter_call_expr(expr::Expr)
    @ JuliaInterpreter ~/.julia/packages/JuliaInterpreter/nxxiG/src/construct.jl:623
 [13] top-level scope
    @ ~/Programa/scratch.jl:753
 [14] include(fname::String)
    @ Main ./sysimg.jl:38
 [15] top-level scope
    @ REPL[2]:1
in expression starting at /Users/cedric/Programa/scratch.jl:14

The reason for failure is quite simply that the file is nothing

julia> dump(@macroexpand @outer)
Expr
  head: Symbol block
  args: Array{Any}((1,))
    1: LineNumberNode
      line: Int64 0
      file: Nothing nothing

while the FrameCode constructor assumes that file is a Symbol, hence the MethodError. It'd be quite easy to fix with a lt.file === nothing && continue PR. Shall I do that? I could also fix MacroTools (I wrote the @qq macro) if someone makes the case that MacroTools' output is wrong.