Compute type tooltips better for macro expansions

Macros often expand into forms where the subforms have
the same syntax location as the entire form, which means
a naive approach gives confusing type tooltips on macro
expressions.

Instead, only keep the latest (outermost) type tooltip
for a given expression.

We could instead compute whether a given piece of syntax
is a subform of another at a given location or not, but
that check is expensive and we don't want to do that on
every update of the type table.