[flang] Catch calls to impure intrinsics from PURE subprograms (#160947)

The code in expression semantics that catches a call to an impure
procedure in a PURE context misses calls to impure intrinsics, since
their designators have a SpecificIntrinsic rather than a Symbol. Replace
the current check with a new one that uses the characteristics of the
called procedure, which works for both intrinsic and non-intrinsic
cases.

Testing this change revealed that an explicit INTRINSIC statement wasn't
doing the right thing for extension "dual" intrinsics that can be called
as either a function or as a subroutine; the use of an INTRINSIC
statement would disallow its use as a subroutine. I've fixed that here
as well.

Fixes https://github.com/llvm/llvm-project/issues/157124.

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