Common/FileUtil: Use std::string_view with WriteStringToFile

Allows writing out other forms of strings (e.g. C strings) without the
need to allocate a std::string and discard it after use.

Source/Core/Common/FileUtil.cpp

@@ -887,7 +887,7 @@ std::string GetThemeDir(const std::string& theme_name)
   return GetSysDirectory() + THEMES_DIR "/" DEFAULT_THEME_DIR "/";
 }
 
-bool WriteStringToFile(const std::string& filename, const std::string& str)
+bool WriteStringToFile(const std::string& filename, std::string_view str)
 {
   return File::IOFile(filename, "wb").WriteBytes(str.data(), str.size());
 }