[clang][bytecode] Fix crash on discarded complex comparison (#177731)

Fixes llvm#176902: [clang][bytecode] crashes on ill-formed
_Static_assert comparing complex value

This patch resolves a crash in Clang's constant evaluation when handling
complex number comparisons in discarded expressions, such as those
involving short-circuiting logical operators. The crash occurred due to
unnecessary evaluation of the comparison in the experimental constant
interpreter.

The issue was originally observed and minimized in the following
example:

```cpp
#define EVAL(a, b) _Static_assert(a == b, "")

void foo() {
  EVAL(; + 0, 1i);
}
```

This patch ensures that such comparisons are handled correctly without
triggering assertions when the result is discarded.

Tests

A regression test has been added to verify that complex comparisons in
discarded expressions no longer cause crashes during constant
evaluation.

Local verification:


llvm-project-main/clang/lib/AST/ByteCode/constant-eval-complex-discard.c
passes.

Full ninja check-clang may fail locally due to a known GCC ICE when
building Clang unittests; this is unrelated to the change itself.