[flang] Fix problems with extended derived types

When looking for a component name in a derived type, also look in the
parent type. Before adding a component to a derived type, report an
error if it already has one with that name. Check that components are
accessible when they are accessed.

Add the "parent component" to derived types (i.e. a component with the
same name as the parent type). The symbol is marked with the
`ParentComp` flag so we can avoid writing it to `.mod` files.

Add calls to `add_occurrence()` so that those particular instances of
`parser::Name` get their symbol set.

Change `DeclareObjectEntity` and `DeclareProcEntity` to use `SourceName` as
the name passed in rather than `parser::Name`.

Fix some problems in `unparse-with-symbols.cc` on statements that both
define and reference names.

Fixes flang-compiler/f18#187.

Original-commit: flang-compiler/f18@664b9627f2be6dc4212f6904e6f14ee642b473a8
Reviewed-on: https://github.com/flang-compiler/f18/pull/192
Tree-same-pre-rewrite: false