[flang] Implement derived type description table encoding

[flang] Implement derived type description table encoding

Define Fortran derived types that describe the characteristics
of derived types, and instantiations of parameterized derived
types, that are of relevance to the runtime language support
library.  Define a suite of corresponding C++ structure types
for the runtime library to use to interpret instances of the
descriptions.

Create instances of these description types in Semantics as
static initializers for compiler-created objects in the scopes
that define or instantiate user derived types.

Delete obsolete code from earlier attempts to package runtime
type information.

Differential Revision: https://reviews.llvm.org/D92802