add bunch of fixmes: currently there exist some functions that accept LLVMValueRef, some that accept LLVMMetadataRef, and replacing one with another not always possible without explicit convertion

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -1150,6 +1150,8 @@ extern "C" void LLVMRustWriteValueToString(LLVMValueRef V,
 }
 
 // LLVMArrayType function does not support 64-bit ElementCount
+// FIXME: replace with LLVMArrayType2
+// https://github.com/llvm/llvm-project/commit/35276f16e5a2cae0dfb49c0fbf874d4d2f177acc
 extern "C" LLVMTypeRef LLVMRustArrayType(LLVMTypeRef ElementTy,
                                          uint64_t ElementCount) {
   return wrap(ArrayType::get(unwrap(ElementTy), ElementCount));