[flang] Handle typeless (BOZ) arguments in AreCompatibleTypes()

The current code can crash due to the representation's use of a negative
INTEGER kind code to signify a typeless (BOZ) argument's "type" as a
DynamicType.  Detect and handle that case, and change some direct
uses of the kind_ data member into kind() accessor references in
places that shouldn't be confronted with BOZ.

Differential Revision: https://reviews.llvm.org/D159023