[flang] Change Scope::name() to Scope::GetName()

Address comments. Not all scopes are related to
a name. This change makes this more visible to compiler
programers by changing `scope:name()` into `Scope::GetName()`
that returns an optional `SourceName` instead of always
returning a `SourceName` and dying when it cannot.

Original-commit: flang-compiler/f18@0addb79919f69a6530b0919356a24e68f21507c1
Reviewed-on: https://github.com/flang-compiler/f18/pull/634
Tree-same-pre-rewrite: false