Fix a hang in type inference after workspace()

[simonster]

The test case to reproduce is:

julia> f(x) = x+1;

julia> workspace()

julia> LastMain.f(2)

The problem is that LastMain is its own parent, but it isn't jl_main_module, and istopmod is apparently false, so this was evidently an infinite loop. I'm not sure if this is the right fix, but I can build Julia with it after make clean, and the code above no longer hangs. Ref #11274, cc @vtjnash

I will also try to add a test for this.

[vtjnash]

lgtm