This commit is a slightly modified revert of 4f29c113.
IS_PRE_RELEASE allows to make LATEST_STABLE version behave as
experimental when this flag is set to true.
The general goal is to prepare fix of KT-62058; after this commit
one can do it by changing IS_PRE_RELEASE flag to true.
The fix of KT-62058 is planned to be done during bootstrapping.
This preparation and the future fix are parts of umbrella KT-61951.
Now, milestones of major Kotlin releases (e.g. 1.6.0-M1) will NOT
generate prerelease binaries anymore.
The reason for that is it's proven to be quite a complicated process to
turn on IR_PRE_RELEASE after the release is branched, perform double
bootstrap and fix tests, and then do it again in reverse just before
release. With the new release cadence, we don't have that much time to
do it and verify that everything works as intended.
Note that this only removes the "global" prerelease flag. Compiler will
still generate prerelease binaries if a non-stable language version is
used. For example, Kotlin 1.6.0-M1 with `-language-version 1.7` will
generate prerelease binaries.
Mikhail Glukhikh