Crash regression with completion starting with 0.30.0-insiders

[sean-mcmanus]

See the repro at #5864 , i.e. the code at https://pytorch.org/cppdocs/frontend.html
do completion on the "forward" of "fc1->forward".

We need to see if this repros on Windows and with VS.

[sean-mcmanus]

I've filed VS bug https://developercommunity.visualstudio.com/content/problem/1170465/c-intellisense-crashes-from-completion-in-clang-mo.html .