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

[via cfe-commits]

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

This is to address https://github.com/llvm/llvm-project/issues/18763

it removes warnings from using a signed char as an array bound if the char is a known positives constant.

This goes one step farther than gcc does.

For example given the following code
```c++
char upper[300];

int main() {
  upper['a'] = 'A';
  char b = 'a';
  upper[b] = 'A';
  const char c = 'a';
  upper[c] = 'A';
  constexpr char d = 'a';
  upper[d] = 'A';
  char e = -1;
  upper[e] = 'A';
  const char f = -1;
  upper[f] = 'A';
  constexpr char g = -1;
  upper[g] = 'A';
  return 1;
}
```

clang currently gives warnings for all cases, while gcc gives warnings for all cases except for 'a' (https://godbolt.org/z/5ahjETTv3)

with the change there is no longer any warning for 'a', 'c', or 'd'.

```
../test.cpp:7:8: warning: array subscript is of type 'char' [-Wchar-subscripts]
    7 |   upper[b] = 'A';
      |        ^~
../test.cpp:13:8: warning: array subscript is of type 'char' [-Wchar-subscripts]
   13 |   upper[e] = 'A';
      |        ^~
../test.cpp:15:8: warning: array subscript is of type 'char' [-Wchar-subscripts]
   15 |   upper[f] = 'A';
      |        ^~
../test.cpp:17:8: warning: array subscript is of type 'char' [-Wchar-subscripts]
   17 |   upper[g] = 'A';
      |        ^~
../test.cpp:15:3: warning: array index -1 is before the beginning of the array [-Warray-bounds]
   15 |   upper[f] = 'A';
      |   ^     ~
../test.cpp:1:1: note: array 'upper' declared here
    1 | char upper[300];
      | ^
../test.cpp:17:3: warning: array index -1 is before the beginning of the array [-Warray-bounds]
   17 |   upper[g] = 'A';
      |   ^     ~
../test.cpp:1:1: note: array 'upper' declared here
    1 | char upper[300];
      | ^
6 warnings generated.
```

This pull request in incomplete in that this is my first change submitted and I don't know how to add tests, any guidance on what sort of tests to add, and where to find documentation on the testing infrastructure  

>From b3111e99b8caf875bca6c59c5ac78cfc0a9f0d0f Mon Sep 17 00:00:00 2001
From: Brian Wheatman <bwheatman at gmail.com>
Date: Sat, 14 Oct 2023 12:02:19 -0400
Subject: [PATCH] Remove warnings from -Wchar-subscripts for known positive
 constants

---
 clang/lib/Sema/SemaExpr.cpp | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index aa30a3a03887558..dd9ba5cecaf2404 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -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();
+    }
+  }
 
   // C99 6.5.2.1p1: "shall have type "pointer to *object* type". Similarly,
   // C++ [expr.sub]p1: The type "T" shall be a completely-defined object