Replaced createTemporaryVariable usages with createTmpVariables.
The latter doesn't rely on KotlinType's. Using KotlinType with
JS_IR backend causes runtime excpetions, as some expected descriptor
API is not implemented, because it avoids descriptors as much as
possible.
Move kotlin-stdlib-js project and the sources specific to the current backend to 'stdlib/js-v1' directory,
but leave sources that can be shared with the new IR backend in the common 'stdlib/js' location
with exception for 'stdlib/js/src/generated', which is used exclusively for current backend.
This simplifies sourceset configuration when building stdlib with the new backend.