[flang] Downgrading the semantics check for C1127 to a warning.

This check to see if there are two DEFAULT (NONE) clauses in a DO CONCURRENT
header.

Original-commit: flang-compiler/f18@9b234125fa89b4d0fb6718a81215f04d696bfece
Reviewed-on: https://github.com/flang-compiler/f18/pull/513