[analyzer] Fix stdin declaration in C++ tests (#66074)

The `stdin` declaration should be within `extern "C" {...}`, in C++
mode. In addition, it should be also marked `extern` in both C and
C++ modes.

I tightened the check to ensure we only accept `stdin` if both of these
match. However, from the Juliet test suite's perspective, this commit
should not matter.

https://github.com/llvm/llvm-project/pull/66074