lupdate: Use clang to parse C++ code

lupdate uses a custom C++ parser to look up and extract translations from source code. This has seen only smaller updates since Qt 4, and has some issues with some modern C++ constructs.

We should consider switching to clang instead, like we already did for qdoc.

Advantages

• Support for modern C++
• Support for Objective-C, Objective-C++

Disadvantages

• Translations in code that is conditionally compiled might get ignored, and would need adaptations

Risks

• Speed - clang is significantly slower than the current parser
• Clang is less robust , and might fail to parse code that the old parser does accept