[fir] Add utility function to FIRBuilder and MutableBox

This patch is extracted from D111337 to make is smaller.
It introduce utility functions to the FIRBuilder and add the MutableBox
files.

- genShape
- readCharLen
- getExtents

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D112207



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>