[flang] Clean up; fix assert and crashes

[flang] Clean up; fix assert and crashes

This is the result of running `f18 -fdebug-resolve-names` on a bunch of
Fortran source and fixing the resulting assertion errors and segmentation
faults.

Most of the problems were with encountering attributes, array specs, or
declaration type specs when we weren't prepared to handle them. Those
were fixed by adding calls to {Begin,End}DeclTypeSpec and {Begin,End}Decl
when encountering certain nodes, though the real work for those nodes is
still to be done.

Extract some common functionality into PostAttrSpec and CheckUseError.

Add missing XOR to GenericSpec.

Original-commit: flang-compiler/f18@3c0ff7ded4b3f5512a03e994edd8a0d5074e1149
Reviewed-on: https://github.com/flang-compiler/f18/pull/93
Tree-same-pre-rewrite: false