common: Silence two discarded result warnings

These are intentionally discarded internally, since the rest of the
public API allows querying success. We want all non-internal uses of
these functions to be explicitly checked, so we can signify that we
intentionally want to discard the return values here.

src/common/file_util.cpp

@@ -915,11 +915,11 @@ std::string SanitizePath(std::string_view path_, DirectorySeparator directory_se
     return std::string(RemoveTrailingSlash(path));
 }
 
-IOFile::IOFile() {}
+IOFile::IOFile() = default;
 
 IOFile::IOFile(const std::string& filename, const char openmode[], int flags)
     : filename(filename), openmode(openmode), flags(flags) {
-    Open();
+    void(Open());
 }
 
 IOFile::~IOFile() {