[flang] These changes are in response to comments on my pull request for C1123.

Here's a summary of the changes:
 - I renamed a function to ResolveControlExpressions for clarity.
 - I changed the interface to the function to ResolveIndexName for clarity.
 - I simplified the code for walking the "step" and scalar mask expressions in
   a DO CONCURRENT control statements to not check for the presence of optional
   expressions.
 - I added "const" to the declaration of a local variable.
 - I removed some spurious comments from the test dosemantics04.f90.
 - I added two new tests to dosemantics04.f90 to verify cases that were not
   covered in my original test implementation.

Original-commit: flang-compiler/f18@fa5827061a4cfdf6a0f8db389202d7657dffdddd
Reviewed-on: https://github.com/flang-compiler/f18/pull/504
Tree-same-pre-rewrite: false