[fir] Move parser/printer/verifier of fir.string_lit and add builders

Move the parser, printer and verifier to the .cpp file. Add builders
needed for lowering.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: schweitz, mehdi_amini

Differential Revision: https://reviews.llvm.org/D110686



Co-authored-by: Valentin Clement <clementval@gmail.com>