[flang] Implement host association and import statements

Host association is done by search for symbols using `Scope::FindSymbol()`
which looks for the the name in the parent scope if the import rules
permit it.

Import statements are implemented using `importKind_` and `importNames_`
in class `Scope`. Most of the constraints are checked when the
`ImportStmt` is encountered. `CheckImports()` is called at the end of
the `SpecificationPart` to verify the names mentioned in the IMPORT
statement. That has to happen then so that we can detect if an imported
name is hidden by a declaration in the current scope.

Original-commit: flang-compiler/f18@0d50c8a8ba12692e6246c4e1a8a098f2850aba3a
Reviewed-on: https://github.com/flang-compiler/f18/pull/167
Tree-same-pre-rewrite: false