[flang][NFC] rename isAbsent to isStaticallyAbsent in IntrinsicCall.cpp

isAbsent/isPresent helpers only give information about static presence
of intrinsic arguments. Many intrinsic arguments optionality is dynamic
(an absent dummy can legally be passed to these intrinsics). This
requires a different handling (like `handleDynamicOptional`).

Rename the helpers to avoid misleading coder/reader into thinking all
optionality cases are covered by them.

Differential Revision: https://reviews.llvm.org/D123378