SE-0458: Disambiguate postfix expressions vs. unsafe expressions more generally
Handle call-vs-tuple and subscript-vs-collection-expr disambiguation using the
same "no trivia" rule that we used to disambiguite "unsafe.x" (which we
treat as a member access) from "unsafe .x" (which we treat as an unsafe
expression with a leading-dot member access).
Fixes rdar://146459104.