[clang] [Clang] Introduce OverflowBehaviorType for fine-grained overflow control (PR #148914)

[Justin Stitt via cfe-commits]

================
@@ -1480,6 +1480,14 @@ class DeclRefExpr final
     return DeclRefExprBits.IsImmediateEscalating;
   }
 
+  bool isOverflowBehaviorDiscarded() const {
----------------
JustinStitt wrote:

The purpose is to emit a different diagnostic for when an OBT is lost during assignment.

```c
int __no_wrap a = 1;
int b = a + 13;

warning: implicit conversion from '__no_wrap int' to 'int' during assignment discards overflow behavior [-Wimplicit-overflow-behavior-conversion-assignment]
   11 |   int b = a + 13;
      |       ~   ~~^~~~
```

I've reworked this slightly in [8c875bb](https://github.com/llvm/llvm-project/pull/148914/commits/8c875bb1750c1f5fb1ac37ead58fb046501dd9f2). I moved the flag from `DeclRefExprBits` to `ExprBits` and added a comment. I also simplified the logic in `CheckOverflowBehaviorTypeConversion`.

https://github.com/llvm/llvm-project/pull/148914