[clang] [Clang] avoid adding consteval condition as the last statement to preserve valid CFG (PR #116513)

[Aaron Ballman via cfe-commits]

================
@@ -5,3 +5,9 @@ static_assert(__is_same(decltype([] constexpr -> int { }( )), int)); // expected
 
 consteval int g() { } // expected-warning {{non-void function does not return a value}}
 static_assert(__is_same(decltype([] consteval -> int { }( )), int)); // expected-warning {{non-void lambda does not return a value}}
+
+namespace GH116485 {
+int h() {
+    if consteval { }
+} // expected-warning {{non-void function does not return a value}}
----------------
AaronBallman wrote:

Other test cases to add are:
```
void foo(int x) {
    if consteval {
    }
    switch (x) {
    case 1:
      foo(12); // Should get a `-Wimplicit-fallthrough` warning here
    case 2:
      break;
    }
}
```
https://godbolt.org/z/rns3Yjofq

or testing whether the consteval if actually causes an early return:
```
constexpr bool foo()  {
    if !consteval { return true; }
}

bool b = foo();
constinit bool c = foo();
```

https://github.com/llvm/llvm-project/pull/116513