[flang] Fold LGE/LGT/LLE/LLT intrinsic functions

Fold the legacy intrinsic functions LGE, LGT, LLE, & LLT
by rewriting them into character relational expressions and
then folding those.  Also fix folding of comparisons of
character values of distinct lengths: the shorter value must
be padded with blanks.  (This fix exposed some bad test cases,
which are also fixed.)

Differential Revision: https://reviews.llvm.org/D111843