Optional symbols keep their declared type

[ahejlsberg]

With this PR, symbols for optional parameters, properties, and methods keep their actual declared type instead of having undefined added to the type when in --strictNullChecks mode. The undefined is added to the type in a later phase when checking expressions involving the parameter or property (specifically by checkIdentifier and checkPropertyAccessExpression). This change allows us to better preserve fidelity in declaration file emitting and also makes it possible to have different behavior for reading vs. writing of optional parameters and properties. For example, the following is now an error:

function foo(x?: number) {
    x = undefined;  // Error, declared type doesn't include undefined
}

Fixes ##8681.

[ahejlsberg]

Thinking about this issue some more, I'm going to close this PR and put up a more localized solution.