[SYCL] Update sycl complex implementation (#8068)

This PR updates the complex implementation.

From the POV of the complex's user, the interface does not change.

The modification made to the implementation are:
- the operators are now declared as hidden friends
- encapsulation of helper functions of the implementation (only expose
what needs to be)
- using metaprogramming to specialize the complex when the trait
`is_genfloat` is true, which removes the duplication of the operators
- using metaprogramming to specialize different math operations when the
trait `is_genfloat` is true, which removes some boilerplate code
- add the missing `inline` attribute to different functions
- remove unnecessary const and ref keywords on `value_type`
- add missing `std::enable_if` on free functions

llvm-test-suite: intel/llvm-test-suite#1533

Author: jle-quel