[flang] Use definability tests for better PURE constraint checking

Many semantic checks for constraints related to PURE subprograms
can be implemented in terms of Semantics' "definable.h" utilities,
slightly expanded.  Replace some particular PURE constraint
checks with calls to WhyNotDefinable(), except for cases that
had better specific error messages, and start checking some
missing constraints with DEALLOCATE statements and local
variable declarations.

Differential Revision: https://reviews.llvm.org/D147389