[flang] Correct bug in literal CHARACTER constant names

The names of CHARACTER strings were being truncated leading to invalid
collisions and other failures. This change makes sure to use the entire
string as the seed for the unique name.

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

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D128884



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