[clang] [Clang][attr] Add 'cfi_salt' attribute (PR #141846)

[Aaron Ballman via cfe-commits]

================
@@ -7941,6 +7942,32 @@ static bool handleFunctionTypeAttr(TypeProcessingState &state, ParsedAttr &attr,
     return true;
   }
 
+  if (attr.getKind() == ParsedAttr::AT_CFISalt) {
+    if (attr.getNumArgs() == 0)
+      return true;
+
+    // Delay if this is not a function type.
+    StringRef Argument;
+    if (!S.checkStringLiteralArgumentAttr(attr, 0, Argument))
+      return false;
+
+    // Delay if this is not a function type.
+    if (!unwrapped.isFunctionType())
+      return false;
+
+    const auto *FnTy = unwrapped.get()->getAs<FunctionProtoType>();
+    if (!FnTy)
----------------
AaronBallman wrote:

I think what's being asked for is a test case like:
```
// RUN: %clang_cc1 -std=c89 -fsyntax-only -fsanitize=kcfi -verify %s

// K&R C function without a prototype
void func() __attribute__((cfi_salt("pepper"))); // expected-error {{attribute only applies to function types with a prototype}}

void (*fp)() __attribute__((cfi_salt("pepper")));  // expected-error {{attribute only applies to function pointer types with a prototype}}
```
(or something along those lines). As it stands, it looks like this code will silently drop the attribute because `getAs` will return null and we'll early return here.

https://github.com/llvm/llvm-project/pull/141846