[libc][NFC] reorganize structs in printf

Previously the type description structs were defined in the parser. For
the fuzzing targets we'll need to use those, so I've moved them into
core_structs.h. Additionally I've renamed the function for determining
the TypeDesc from a given type. Previously it shared its name with
get_type_desc which is a related but separate function that is a part of
the parser.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D143595