[clang] [clang][ASTImporter] Remove trailing return testing on lambda proto (PR #101031)

[Ding Fei via cfe-commits]

danix800 wrote:

> > Another PR would fix the crash when this one is landed.
> 
> Can you land the fix for the crash first? If the other PR gets reverted it creates a lot of churn, so it would preferable to land that first. Assuming it is not somehow dependent on this fix.

This new crash is a false alarm caused by the first incorrect fix. Sorry about that!

Could you also take a look at the new fix for the first crash?

Previously we think that lambda without trailing return couldn't
have return type defined inside its body, but for
```c
  [] { // no parentheses for parameters
    struct X {};
    return X();
  };
```
the proto has trailing return deduced to the type defined inside.

So I think we should remove the trailing return test for lambdas.

https://github.com/llvm/llvm-project/pull/101031