Lambda and local function suppressions (#2689)

This adds support for RequiresUnreferencedCode and UnconditionalSuppressMessage on lambdas and local functions, relying on heuristics and knowledge of compiler implementation details to detect lambdas and local functions.

This approach scans the code for IL references to lambdas and local functions, which has some limitations.

- Unused local functions aren't referenced by the containing method, so warnings from these will not be suppressed by suppressions on the containing method. Lambdas don't seem to have this problem, because they contain a reference to the generated method as part of the delegate conversion.
- The IL doesn't in general contain enough information to determine the nesting of the scopes of lambdas and local functions, so we make no attempt to do this. We only try to determine to which user method a lambda or local function belongs. So suppressions on a lambda or local function will not silence warnings from a nested lambda or local function in the same scope.

This also adds warnings for reflection access to compiler-generated state machine members, and to lambdas or local functions. For these, the analyzer makes no attempt to determine what the actual IL corresponding to the user code will be, so it produces fewer warnings. The linker will warn for what is actually in IL.

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