[flang] Don't compute pointer component procedure characteristics when not needed

When a procedure pointer component has an interface that is a forward
reference to a procedure, syntax errors can be emitted if there is
a structure constructor that tries to initialize that component,
since its characteristics are not yet known; however, when the
initializer is a bare NULL(with no mold), those characteristics
don't matter.  Make the characterization of the procedure pointer
component take place only when needed.

Differential Revision: https://reviews.llvm.org/D131100