[flang] Add common::ImportKind

Replace parser::ImportStmt::Kind and semantics::Scope::ImportKind with a
single enum class in common/fortran.h. This eliminates the need to map
between the parser enum and the semantics enum.

Original-commit: flang-compiler/f18@779a99314c69c44e5a76cd9bd3d1833b870ef348
Reviewed-on: https://github.com/flang-compiler/f18/pull/167