Added checking for completeness of lvalue in conditional operator

Given:
```
struct x y;
int main(void)
{
    (void)(1 ? y : y);
}
struct x {int i;};
```
The conditional operator(?:) requires the second and third operands to
be of compatible types. To be compatible, they also need to be
complete (however, both can be void). Therefore, the expected response
from clang after running the above code as a C program should be error
dialogue pointing out that both the types are incomplete hence
incompatible, but the code compiled without any errors.

The patch ensures the completeness in the CheckCondtionalOperand
function present in llvm-project/clang/lib/Sema/SemaChecking.cpp.

Fixes https://github.com/llvm/llvm-project/issues/59718
Differential Revision: https://reviews.llvm.org/D144358