[flang][openmp] Bad code for "private" clause and named common

[psteinfeld]

Here's a reproducer:

Program bug
  Real :: x
  Common /named_common/ x
  x = 1.0
  Call sub()
  If (x/=1.0) Then
    Print *,'FAIL common, x: ',x
  Else
    Print *,'ok'
  End If
contains
  Subroutine sub
    Real :: x
    Common /named_common/ x
    !$OMP PARALLEL PRIVATE (/named_common/)
      x = 2.0
    !$OMP END PARALLEL
  End Subroutine
End Program

If I compile and execute this program with the -fopenmp option, I get the output:

 FAIL common, x:  2.

Alternatively, gfortran produces the output:

 ok

[tblah]

See https://discourse.llvm.org/t/rfc-openmp-fix-issue-in-mlir-to-llvmir-translation-for-delayed-privatisation/81225