Avoid adding two holes to value collection literals
Summary: ValueCollection literals are special cased in typing since we know that all elements must be subtypes of arraykey. However, since a `Hole` doesn't change the type of an expression, typing will later identify another type mismatch and add a second `Hole`. This diff adds a flag to the helper function `compute_exprs_and_supertype` to avoid doing this
Reviewed By: andrewjkennedy
Differential Revision:
D35245209
fbshipit-source-id:
df5cdeb58175016766a49e507433530412323a10