Fix IsPartial check and escaping of assembly name

[manodasanW]

• We previously didn't escape - and @ in the assembly name when using it for an identifier or namespace. This addresses that.
• Our IsPartial check wasn't complete where we didn't also check whether the parent classes were partial too to allow for us to generate source. Due to that, we would generate source in such cases leading to a compiler error rather than raising a diagnostic as we did when the type itself wasn't partial.
• Improve the code fixer to handle making parent types partial.
• Add partial diagnostics for the GeneratedCustomBindableProperty attribute too

Fixes #1799
Fixes #1800

[manodasanW]

Failures are post build infra issues, tests passed.