[flang] Refactoring: improve some names

IntrinsicTypeSpec was used for all intrinsics except for character.
Change it to be a common base class for NumericTypeSpec,
LogicalTypeSpec, and CharacterTypeSpec.

Change DeclTypeSpec to categorize the intrinsics as Numeric, Logical,
and Character. Add some utility methods: AsIntrinsic() and IsNumeric().

In scope.h, give the functions that create DeclTypeSpecs better names.

In semantics.h, replace MakeIntrinsicTypeSpec() with MakeNumericType()
and MakeLogicalType() as it does not apply to character types.

Original-commit: flang-compiler/f18@8ad92d069c5ac56a9610937e5669848b67664203
Reviewed-on: https://github.com/flang-compiler/f18/pull/247
Tree-same-pre-rewrite: false