[llvm-c] Document that LLVMGetElementType on pointers is deprecated (NFC)

We can't actually deprecate the function, because it is also used
for arrays and vectors, so we can only document this.

Author: nikic

llvm/include/llvm-c/Core.h

@@ -1391,9 +1391,9 @@ LLVMBool LLVMIsLiteralStruct(LLVMTypeRef StructTy);
  */
 
 /**
- * Obtain the type of elements within a sequential type.
+ * Obtain the element type of an array or vector type.
  *
- * This works on array, vector, and pointer types.
+ * This currently also works for pointer types, but this usage is deprecated.
  *
  * @see llvm::SequentialType::getElementType()
  */

llvm/lib/IR/Core.cpp

@@ -798,7 +798,7 @@ LLVMTypeRef LLVMScalableVectorType(LLVMTypeRef ElementType,
 LLVMTypeRef LLVMGetElementType(LLVMTypeRef WrappedTy) {
   auto *Ty = unwrap<Type>(WrappedTy);
   if (auto *PTy = dyn_cast<PointerType>(Ty))
-    return wrap(PTy->getPointerElementType());
+    return wrap(PTy->getNonOpaquePointerElementType());
   if (auto *ATy = dyn_cast<ArrayType>(Ty))
     return wrap(ATy->getElementType());
   return wrap(cast<VectorType>(Ty)->getElementType());