[Development] New BiC gotcha: adding noexcept

[Marc Mutz]

Hi,

I just realised: Since C++17, noexcept is part of the mangled name. That 
means that adding noexcept to an exported function is BiC now! Please be 
aware of this when reviewing patches.

Since non-template inline members of exported classes are exported, too, 
in MSVC Debug builds, the same is true for them: You can no longer add 
noexcept to inline functions of exported classes (yet another reason to 
avoid exporting non-polymorphic classes wholesale; individually export 
single functions instead).

Do we need a new macro (Q_DECL_CPP17_COMPATIBLE_NOTHROW?) that expands 
to nothing on MSVC Debug builds?

Or should we simply dodge the issue and declare debug builds of Qt not 
subject to BC on MSVC?

Thanks,
Marc