[ComInterfaceGenerator] Warn if StringMarshalling doesn't match base and warn if base interface cannot be generated

[jtschuster]

Warns if the base interface does not have the same StringMarshalling or if a base interface cannot be generated. Adds a LocationInfo to be able to produce diagnostics for interfaces without the ISymbol or anything that ruins incrementality.

This defines a few diagnostics / behaviors:

• If a base interface fails to be generated, then the inheriting interfaces will not be generated.
• If a base interface defines StringMarshalling, then all inheriting interfaces must match the base StringMarshalling.
• If a base interface does not define StringMarshalling (because it doesn't use strings or uses MarshaAs / MarshalUsing for each), an inheriting interface may define StringMarshalling with no error.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

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Issue Details

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Warns if the base interface does not have the same StringMarshalling or if a base interface cannot be generated. Adds a LocationInfo to be able to produce diagnostics for interfaces without the ISymbol or anything that ruins incrementality.

Still a few TODOs to clean it up, but the main logic likely won't change if you want to take a look.

Author:	jtschuster
Assignees:	jtschuster
Labels:	area-System.Runtime.InteropServices, new-api-needs-documentation
Milestone:	-