Make qt_noop() constexpr and noexcept (for C++)

Since qt_noop() is supposed to be a no-op, it should not affect
constexpr'ness and noexcept'ness of functions and expressions it's
(indirectly) used in.

That requires that it be constexpr and noexcept itself. Add the
keywords, but guard against use from C. We can't use our Q_ macros
(which would enable noexcept for C, too, on some compilers, because
qcompilerdetection.h depends on qtnoop.h, and we don't want the
circular dependency.

Pick-to: 6.6 6.5
Change-Id: Ie5441c423b7942a6c956345126fc7aec99907d25
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qtnoop.h

@@ -8,6 +8,13 @@
 #pragma qt_sync_stop_processing
 #endif
 
-inline void qt_noop(void) {}
+#ifdef __cplusplus
+constexpr
+#endif
+inline void qt_noop(void)
+#ifdef __cplusplus
+    noexcept
+#endif
+{}
 
 #endif // QTNOOP_H