[spike] Find out whether all supported compilers support [[nodiscard]] on ctor, add macro if not

We'd like to apply [[nodiscard]] to our RAII class' ctors (cf. epic for why), and we need to find out whether we need a macro for that in qcompilerdetection.h.

For compilers that don't, check whether they act on __attribute__((__warn_unused_result__)) instead.

Test case:

QMutex mut;
QMutexLocker<QMutex>{&mut}; // Clang already warns b/c of class-level [[nodiscard]], GCC does not

Acceptance criteria:

1. proove that all compilers syntactially support [[nodiscard]] on ctors
   • semantically, it's ok if they ignore it w/o warning
2. or else add a macro Q_NODISCARD_CTOR to qcompilerdetection.h, expanding to
   1. [[nodiscard]]
   2. or __attribute__((__warn_unused_result__))
   3. or nothing, depending on what the particular compiler supports on ctors.
3. add a test (can only be syntax-only, but add nonetheless)

The 5min way to solve this is

#if __has_cpp_attribute(nodiscard) >= 201907L
# define Q_NODISCARD_CTOR [[nodiscard]]
#else
# define Q_NODISCARD_CTOR
#endif

but that would mean it's limited to C++20, while we'd really like this to be available in C++17, too.