RFC: Embed an error when a generator fails.

[maleadt]

Fix #25707, and also results in more GPU compatible code since it doesn't dynamically dispatch to the generator in order to throw an error.

Some thoughts/TODO's:

• Expr(:error, e) doesn't work because jl_expand(Expr(:lambda, ...)) doesn't return a CodeInfo then?
• stack traces are obviously truncated; should we reconstruct the line info stack to make them accurate?

Also, the new behavior is shown by code_typed but not code_warntype, I'm not sure why that's the case (probably #31764):

julia> @generated bar() = whatever
bar (generic function with 1 method)

julia> foo() = bar()
foo (generic function with 1 method)

julia> @code_warntype foo()
Variables
  #self#::Core.Compiler.Const(foo, false)

Body::Union{}
1 ─     Main.bar()
└──     Core.Compiler.Const(:(return %1), false)

julia> @code_typed foo()
CodeInfo(
1 ─     throw(UndefVarError(:whatever))::Union{}
│       $(Expr(:unreachable))::Union{}
│       φ ()::Union{}
└──     $(Expr(:unreachable))::Union{}
) => Union{}

[vchuravy]

How does this handle the case of:

function f(x)
  if @generated
    error()
  else
    x^2
  end
end

[vtjnash]

We could also try to re-invoke the staged function at runtime, and throw the exception at that time, rather than falling back to storing a copy of the error (which might waste memory) as this PR does, or trying to use the non-specialized code as it does now.