[flang] I fixed a problem in C1123. This constraint requires that expressions in a DO

CONCURRENT header not reference names declared in the same header.  To
implement this, I changed the "Pre" function for ConcurrentHeader to walk the
subtree in an order that caused the variables associated with the declared
names to be created before any of the expressions were walked.

I added the test dosemantics04.f90 to test this.

Original-commit: flang-compiler/f18@9f3b55241407649fc848d83c9415fd7d91e3056b
Reviewed-on: https://github.com/flang-compiler/f18/pull/504
Tree-same-pre-rewrite: false