[clang] Remove warnings from -Wchar-subscripts for known positive constants (PR #69061)

[Aaron Ballman via cfe-commits]

================
@@ -6018,9 +6018,15 @@ Sema::CreateBuiltinArraySubscriptExpr(Expr *Base, SourceLocation LLoc,
                      << IndexExpr->getSourceRange());
 
   if ((IndexExpr->getType()->isSpecificBuiltinType(BuiltinType::Char_S) ||
-       IndexExpr->getType()->isSpecificBuiltinType(BuiltinType::Char_U))
-         && !IndexExpr->isTypeDependent())
-    Diag(LLoc, diag::warn_subscript_is_char) << IndexExpr->getSourceRange();
+       IndexExpr->getType()->isSpecificBuiltinType(BuiltinType::Char_U)) &&
+      !IndexExpr->isTypeDependent()) {
+    std::optional<llvm::APSInt> IntegerContantExpr =
+        IndexExpr->getIntegerConstantExpr(getASTContext());
+    if (!(IntegerContantExpr.has_value() &&
+          IntegerContantExpr.value().isNonNegative())) {
+      Diag(LLoc, diag::warn_subscript_is_char) << IndexExpr->getSourceRange();
+    }
----------------
AaronBallman wrote:

```suggestion
    std::optional<llvm::APSInt> IntegerContantExpr =
        IndexExpr->getIntegerConstantExpr(getASTContext());
    if (!IntegerContantExpr.has_value() ||
        IntegerContantExpr.value().isNegative())
      Diag(LLoc, diag::warn_subscript_is_char) << IndexExpr->getSourceRange();
```
This performs extra work for each array subscript in the TU, but because it's limited to only array subscripts with a character type, I don't think the compile-time performance hit should be too bad, so this seems reasonable to me.

https://github.com/llvm/llvm-project/pull/69061