Streamline delegate-typed component parameters

[SteveSandersonMS]

As discussed:

• Stop auto-generation of LHS of lambda
• Fix the "complex content" errors for cases that should be supported.

End goal: ideally, all of the following should be equivalent for a parameter of type Func<string, int>:

• <MyComponent Callback="str => str.Length" />
• <MyComponent Callback="@(str => str.Length)" />
• <MyComponent Callback=@(str => str.Length) />
• <MyComponent Callback=@((str) => { return str.Length; }) />
• <MyComponent Callback=SomeMethod />, given int SomeMethod(string input) => input.Length;
• <MyComponent Callback=@SomeMethod />, given int SomeMethod(string input) => input.Length;
• <MyComponent Callback=@(SomeMethod) />, given int SomeMethod(string input) => input.Length;

That's just out of my head, though, and there may be reasons why some subset of these shouldn't be supported (or it's too disruptive to support them in the very short term).

[SteveSandersonMS]

Fixed in dotnet/blazor#321