[flang] Improve shape & length characterization

Analyze the shape of the result of TRANSFER(ptr,array) correctly
when "ptr" is an array of deferred shape.  Fixing this bug led to
some refactoring and concentration of common code in TypeAndShape
member functions with code in general shape and character length
analysis, and this led to some regression test failures that have
all been cleaned up.

Differential Revision: https://reviews.llvm.org/D95744