[JuliaLowering] Quote generated function body before desugaring (#60710)
This change moves the separation of the generated and non-generated
function body ahead of desugaring using a new `(generated_function
callex gen_body nongen_body)` form. Previously, desugaring would search
for an `(if (generated) gen_ex nongen_ex)` form in the function body,
then create a method with body `(stuff_surrounding_if_expression...
$gen_ex)`. However, our pre-desugaring (and soon AST conversion) alter
`stuff_surrounding_if_expression` in a way we can't correctly convert
back to Expr. This PR handles quoting of the generated body at the same
time everything else is quoted.