[flang] Fixed regression with CDEFINED linkage (#164616)
https://github.com/llvm/llvm-project/pull/162722 introduced a regression
that started creating initializers for CDEFINED variables. CDEFINED
variables cannot have initializers, because their storage is expected
come from elsewhere, likely outside of Fortran. Fixed the regression and
improved the regression test to catch the incorrect initialization case.
Also, based on the code review feedback, made CDEFINED variable
initialization a hard error and updated tests accordingly.
