Have compiler return semantic errors in lambdas even in the presence of syntax errors

[jmarolf]

Today, the compiler does not return semantic errors inside lambdas in the IDE if there are syntax errors. This causes several quick fix scenarios to fail inside lambdas (for example #1239, #1241, and #1744 are all caused by this problem).

Consider the code sample below from issue #1239:

using System.Threading.Tasks;

class C {
    C() {
        Task.Run(() => {
            new C(0)
        });
    }
}

new C(0) will never be squiggled by the IDE because of the lack of a semicolon. This makes quick-fixes that could generate a constructor unable to run because there is no error to trigger them.

By having the compiler return semantic errors in lambdas even in the presence of syntax errors, many more IDE scenarios will "Just Work"

[CyrusNajmabadi]

Note: whenever this is fixed we need to remove the UnboundIdentifiersDiagnosticAnalyzerBase type and ensure all AddUsing tests (and other tests that depend on this type) still continue passing. Right now, this type is a hack at the IDE layer to get and display reasonable semantic errors to the user and to drive our normal features. This should not be something the IDE is responsible for doing. Users should be getting these errors, even in the presence of a syntax error in a lambda, and our features should not have to list for syntax errors to drive semantic features.