[flang] Substrings with lower bound greater than upper bound

According to section 9.4.1, paragraph 3,
 If the starting point is greater than the ending point, the substring has
 length zero

But the compilers code for substring processing was failing a call to `CHECK()`
in this case.  I fixed this by just setting the number of items in the
resulting string to 0 for this situation.

Differential Revision: https://reviews.llvm.org/D87799