cinterop tool should add ExperimentalForeignApi to all generated
declarations, to indicate their experimental status and discourage using
them in public Kotlin API. But the same considerations are applicable
to forward declarations (cnames.*, objcnames.*), which are generated not
by cinterop tool, but directly by the compiler.
This commit adds ExperimentalForeignApi to those compiler-generated
classes.
^KT-58362
This is refactoring in preparation for KT-59764.
Names and layout of forward declarations related classes
was copy-pasted many times over compiler code.
Implementing KT-59764 would require copy-pasting it two more times.
So instead of doing this it was put in single place.
No behaviour changes intended in this commit.
Svyatoslav Scherbina