QDoc: Simplify noexcept note generation

The noexcept note generator has nested conditionals and converts
member positions through std::string, making its existing numbering
harder to follow. Use a range-for with an explicit position counter
and QString, counting even members without a noexcept note.

Pick-to: 6.12
Task-number: QTBUG-151328
Change-Id: Iadb36c365d7e9dc51b1a9ac51f74a8639d300265
Reviewed-by: Topi Reinio <topi.reinio@qt.io>