[isl++] Do not use enum prefix

Instead of declaring a function as:

  inline val plain_get_val_if_fixed(enum dim type, unsigned int pos) const;

we use:

  inline isl::val plain_get_val_if_fixed(isl::dim type, unsigned int pos) const;

The first argument caused the following compile time error on windows:

  "error C3431: 'dim': a scoped enumeration cannot be redeclared as an
  unscoped enumeration"

In some cases it is sufficient to just drop the 'enum' prefix, but for example
for isl::set the 'enum class dim' type collides with the function name
isl::set::dim and can consequently not be referenced. To avoid such kind of
ambiguities in the future we add the isl:: prefix consistently to all types
used.

Reported-by: Michael Kruse <llvm@meinersbur.de>
llvm-svn: 297478