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

[Pete Steinfeld via flang-commits]

psteinfeld wrote:

@klausler, after this update, we're failing a test.  Here's a small reproducer:
```
program main
  character(len=1) :: c
  c = ""
  if (iachar ("")/= 128) STOP 749
end program main
```
With this change, we print out `Fortran STOP: code 749`.  Without the change, we don't print out anything.  All other compilers I tried print nothing.

https://github.com/llvm/llvm-project/pull/76757