string_util: Add U16StringFromBuffer

Qt's QString::toStdU16String doesn't work when compiling against the
latest libstdc++, at least when using Clang. This function effectively
does the same thing as the aforementioned one.

src/common/string_util.h

@@ -44,6 +44,8 @@ bool SplitPath(const std::string& full_path, std::string* _pPath, std::string* _
 
 #endif
 
+[[nodiscard]] std::u16string U16StringFromBuffer(const u16* input, std::size_t length);
+
 /**
  * Compares the string defined by the range [`begin`, `end`) to the null-terminated C-string
  * `other` for equality.