[flang] Handle continuation line edge case (#74751)

For a character literal that is split over more than one source line
with free form line continuation using '&'
at the end of one line but missing the standard-required '&' on the
continuation line, also handle the case of spaces at the beginning of
the continuation line.
    
For example,
    
PRINT *, 'don'&
   't poke the bear'
    
now prints "don't poke the bear", like nearly all other Fortran
compilers do.
    
This is not strictly standard conforming behavior, and the compiler
emits a portability warning with -pedantic.
    
Fixes llvm-test-suite/Fortran/gfortran/regression/continuation_1.f90,
.../continuation_12.f90, and .../continuation_13.f90.