This change only moves code around, no behavior is changed.
Specifically, ir.tree sources containing several declarations are split
into several files: one file per class, and one file for all extension
functions per package (IrDeclarations.kt, IrExpressions.kt,
IrVisitors.kt, IrConstructorCallTypeArguments.kt).
This is useful because after introducing IR tree generator, we can
easily see how generated sources are different from those which were
written manually, since Git will recognize file moves. Also, it will
keep Git history for sources which consisted of one big class + a couple
of extension functions (e.g. IrElementVisitorVoid.kt).
Alexander Udalov
f17fc67683