[polly] Match function definitions and header declarations. NFC.

Ensure that function definitions match their declrations in header
files, even if they have no effect on linking. This includes

 1. Both have the same __isl_* annotations

 2. Both use the same type alias

 3. Remove unused declarations that have no definition

 4. Use explicit polly namespace qualifier for definitions; generally,
    the .cpp file should use at most an anon namespace region since
    only symbols declared in the header file can be accessed from other
    translation units anyway. For defintions that have been declared in
    the header file, the explicit namespace qualifier ensures that both
    match.