QDate: fix C++20 builds with MSVCHEADdev

Commit c4f7dba81e43c11c1e71c19ef7f0b5acad119e60 reworked the C++20
QDate constructors and introduced a helper stdSysDaysToJulianDay()
method. However, the QT_POST_CXX17_API_IN_EXPORTED_CLASS was not used
on this new helper function.
As a result, an application which is built with C++20 but uses a
pre-built QtCore that is built with C++17 cannot call QDate constructors
with the new std::chorono types.

An obvious fix is to add the missing macro to the function.
However, we cannot do it directly, because the function is exported
in C++20 builds, and adding the macro will unexport it.

Use QT6_{DECL,CALL}_NEW_OVERLOAD_TAIL macros to add a new overload of
the stdSysDaysToJulianDay() function and mark it as
QT_POST_CXX17_API_IN_EXPORTED_CLASS.

Keep the old function under the QT_VERSION < Qt 7 check to keep BC.

Amends c4f7dba81e43c11c1e71c19ef7f0b5acad119e60.

Fixes: QTBUG-125610
Pick-to: 6.7
Change-Id: I999354da51ee58c3691dbc1c0351be15a18a268b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

0 files changed, 0 insertions, 0 deletions