[flang] Add support for character type specs

Recognize the various ways of specifying character lengths.

Define CharacterTypeSpec with length and kind and store them in the
current scope, as is done with DerivedTypeSpec (which can also have
length parameters).

Note: IntrinsicTypeSpec is no longer used for characters, so it
should have a different name. Similarly, in DeclTypeSpec::Category,
Intrinsic does not include Character.

Original-commit: flang-compiler/f18@5f847851933b27dd30bd8165d16a7c390e787b9a
Reviewed-on: https://github.com/flang-compiler/f18/pull/247
Tree-same-pre-rewrite: false