[flang] Do not traverse selectors in FindImpureCall and HasVectorSubscript (#84041)

In presence of symbols with AssocEntityDetails in an expression,
`Traverse`, `AnyTraverse`, `AllTraverse`, and `SetTraverse`
automatically visit the selector expression or variable.

This is most often the desired behavior but can be surprising, and was
not correct for FindImpureCall and HasVectorSubscript.

Add a default template option to flag the behavior to someone willing to
use the Traverse helper for a new utility, and set this template to
false for FindImpureCall and HasVectorSubscript.