[flang] Create a temporary of the correct size when lowering SetLength in genarr

This patch creates a temporary of the appropriate length while lowering SetLength.

The corresponding character can be truncated or padded if necessary.

This fix issue with array constructor in argument and also with statement function.

D132464 was fixing the same issue in genval.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D132866