Sema: Use the right type for PredefinedExpr when it's in a lambda.

1. We now print the return type of lambdas and return type deduced functions
as "auto". Trailing return types with decltype print the underlying type.
2. Use the lambda or block scope for the PredefinedExpr type instead of the
parent function. This fixes PR16946, a strange mismatch between type of the
expression and the actual result.
3. Verify the type in CodeGen.
4. The type for blocks is still wrong. They are numbered and the name is not
known until CodeGen.