[flang][OpenMP] Allow flush of common block

I think this was denied by accident in 68180d8.

Flush of a common block is allowed by the standard on my reading. It is
not allowed by classic-flang but is supported by gfortran and ifx.

This doesn't need any lowering changes. The LLVM translation ignores the
flush argument list because the openmp runtime library doesn't support
flushing specific data.

Depends upon #139522. Ignore the first commit in this PR.

Author: tblah

flang/lib/Semantics/check-omp-structure.cpp

@@ -2304,13 +2304,6 @@ void OmpStructureChecker::Leave(const parser::OpenMPFlushConstruct &x) {
   auto &flushList{std::get<std::optional<parser::OmpArgumentList>>(x.v.t)};
 
   if (flushList) {
-    for (const parser::OmpArgument &arg : flushList->v) {
-      if (auto *sym{GetArgumentSymbol(arg)}; sym && !IsVariableListItem(*sym)) {
-        context_.Say(arg.source,
-            "FLUSH argument must be a variable list item"_err_en_US);
-      }
-    }
-
     if (FindClause(llvm::omp::Clause::OMPC_acquire) ||
         FindClause(llvm::omp::Clause::OMPC_release) ||
         FindClause(llvm::omp::Clause::OMPC_acq_rel)) {

flang/test/Lower/OpenMP/flush-common.f90

@@ -0,0 +1,13 @@
+! RUN: %flang_fc1 -fopenmp -emit-hlfir -o - %s | FileCheck %s
+
+! Regression test to ensure that the name /c/ in the flush argument list is
+! resolved to the common block symbol and common blocks are allowed in the
+! flush argument list.
+
+! CHECK: %[[GLBL:.*]] = fir.address_of({{.*}}) : !fir.ref<!fir.array<4xi8>>
+  common /c/ x
+  real :: x
+! CHECK: omp.flush(%[[GLBL]] : !fir.ref<!fir.array<4xi8>>)
+  !$omp flush(/c/)
+end
+