[flang] Support \u Unicode escape sequences (#76757)

Support \uNNNN and \uNNNNNNNN escape sequences for CHARACTER(KIND=2) and
CHARACTER(KIND=4) literal constants for better GNU Fortran
compatibility.

Fixes llvm-test-suite/Fortran/gfortran/regression/achar_6.F90 and
.../widechar_1.f90.