QT_INLINE_SINCE: never inline for static Qt builds

Static Qt builds are not covered by BC guarantees, but since all
'libraries' are linked into a single executable, we face in all users
of the library the same ODR violation that we faced within the
implementation DLL: we can't define the same symbol as inline in some
TUs and out-of-line in others.

In the past, we decided to always inline in static builds, but that
breaks users which, by Hyrum's Law, have come to depend on the
non-existent BC guarantees for static Qt builds.

By switching to never inline¹ in static builds, we restore BC for such
users. The performance issues should be minimal, since LTO will anyway
inline whatever it wants, independent on how it was declared.

¹ except when the deprecation point has passed over the Qt version the
  API was inlined (-disable-deprecated-up-to configure switch).

[ChangeLog][QtCore] Restored binary compatibility for static Qt
builds broken by the QT_INLINE_SINCE mechanism. Qt still does not
guarantee BC for static build configurations otherwise.

Fixes: QTBUG-109449
Pick-to: 6.5 6.4
Change-Id: Ie3fa62621b74dc5e9dac301b9882c0e3c3999eaf
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

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