Stop keeping everything in types with non-understood DebuggerDisplayAttribute

The linker tries to figure out which are the members referenced by the argument of a `DebuggerDisplayAttribute` by performing regex-match on strings inside curly braces.
Whenever the argument is not understood by the linker, the entire type where the attribute annotation was found is kept -- note that this doesn't necessarily save us from removing things which might be needed when debugging, since you can reference members on _any_ type. This ends up keeping more than what is strictly necessary, since strings within curly braces in a `DebuggerDisplayAttribute` can only refer to [fields, properties or methods](https://docs.microsoft.com/en-us/visualstudio/debugger/using-the-debuggerdisplay-attribute?view=vs-2019).