[flang] Catch more initialization errors (#77850)

[flang] Catch more initialization errors
    
    Diagnose some error cases related to initialization that are
slipping past semantic checking: don't allow multiple initializations
    of the same symbol, and don't allow an object that was initialized
    as a scalar to become an array afterward.
    
Fixes llvm-test-suite/Fortran/gfortran/regression/initialization_17.f90.