Fix return value of `LLVMRustMetadataAsValue`

`LLVMRustMetadataAsValue` would previously return `void`, despite the
corresponding Rust function expecting to return a `ValueRef`.

src/rustllvm/RustWrapper.cpp

@@ -566,8 +566,8 @@ extern "C" void LLVMRustAddModuleFlag(LLVMModuleRef M, const char *Name,
   unwrap(M)->addModuleFlag(Module::Warning, Name, Value);
 }
 
-extern "C" void LLVMRustMetadataAsValue(LLVMContextRef C, LLVMMetadataRef MD) {
-  wrap(MetadataAsValue::get(*unwrap(C), unwrap(MD)));
+extern "C" LLVMValueRef LLVMRustMetadataAsValue(LLVMContextRef C, LLVMMetadataRef MD) {
+  return wrap(MetadataAsValue::get(*unwrap(C), unwrap(MD)));
 }
 
 extern "C" LLVMRustDIBuilderRef LLVMRustDIBuilderCreate(LLVMModuleRef M) {