[flang] Refine error checking in specification expressions

The rules in the Fortran standard for specification expressions
are full of special cases and exceptions, and semantics didn't get
them exactly right.  It is valid to refer to an INTENT(OUT) dummy
argument in a specification expression in the context of a
specification inquiry function like SIZE(); it is not valid to
reference an OPTIONAL dummy argument outside of the context of
PRESENT.  This patch makes the specification expression checker
a little context-sensitive about whether it's examining an actual
argument of a specification inquiry intrinsic function or not.

Differential Revision: https://reviews.llvm.org/D125131