[flang] Fix problems in PR 109 when compiling with clang.

[flang] Fix problems in PR 109 when compiling with clang.

This gets the code base back to compiling cleanly with clang after
pull request 109.

There were two overloadings of `Post(const parser::DeclarationTypeSpec::Type &)`.
The one in DeclarationVisitor needed to call the one in DeclTypeSpecVisitor.
This was fixed by introducing a new function, SetDerivedDeclTypeSpec, to do
the equivalent thing.

Original-commit: flang-compiler/f18@81e447bf4ec076f8fa823c0c61c9dd72e1f62fe4
Reviewed-on: https://github.com/flang-compiler/f18/pull/110