[SYCL][COMPLEX] Split complex header file and tests into multiple files (#11600)

This PR is the first step toward restructuring the `complex` API.

As discussed in #8647 and
#10854, the goal here is to clean the
future-to-be `complex` API.

To simplify the review of these changes, a new PR (this PR) is created
to not overload #8647.
IF/WHEN this will be approved, the second step which does the same for
the `marray`'s complex specialization will be pushed onto
#8647

Here's an overview of what has changed:

For the users, the only change is the include.
Instead of including `<sycl/ext/oneapi/experimental/sycl_complex.hpp>`,
it will now be `<sycl/ext/oneapi/experimental/complex/complex.hpp>`

The `complex.hpp` header files include all the `complex/detail` header
file (the `complex` API), in order to abstract the headers needed for
the users.

However, the users can include (if necessary) the specific component of
the API located in `complex/detail`.

Here's the overview of what the complex directory will contain when the
whole API is merged.

```
- complex
  - complex.hpp
  - detail
    - complex.hpp
    - complex_math.hpp
    - complex_group_algorithm.hpp
    - marray.hpp
    - marray_math.hpp
    - marray_group_algorithm.hpp
    - common.hpp
```

Finally, here's the overview of the `sycl/test/extension`

```
- complex
  - complex.cpp
  - marray.cpp
```

Author: jle-quel

sycl/include/sycl/ext/oneapi/experimental/complex/complex.hpp

@@ -0,0 +1,16 @@
+//===- complex.hpp --------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#pragma once
+
+#ifdef SYCL_EXT_ONEAPI_COMPLEX
+
+#include "./detail/complex.hpp"
+#include "./detail/complex_math.hpp"
+
+#endif // SYCL_EXT_ONEAPI_COMPLEX

sycl/include/sycl/ext/oneapi/experimental/complex/detail/common.hpp

@@ -0,0 +1,62 @@
+//===- common.hpp ---------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#pragma once
+
+#include <type_traits>
+
+#include <sycl/half_type.hpp>
+
+namespace sycl {
+inline namespace _V1 {
+
+namespace ext {
+namespace oneapi {
+namespace experimental {
+
+////////////////////////////////////////////////////////////////////////////////
+/// FORWARD DECLARATIONS
+////////////////////////////////////////////////////////////////////////////////
+
+template <class _Tp> struct is_genfloat;
+
+template <class _Tp, class _Enable = void> class complex;
+
+template <class _Tp>
+class complex<_Tp, typename std::enable_if_t<is_genfloat<_Tp>::value>>;
+
+////////////////////////////////////////////////////////////////////////////////
+/// TRAITS
+////////////////////////////////////////////////////////////////////////////////
+
+template <class _Tp>
+struct is_genfloat
+    : std::integral_constant<bool, std::is_same_v<_Tp, double> ||
+                                       std::is_same_v<_Tp, float> ||
+                                       std::is_same_v<_Tp, sycl::half>> {};
+
+template <class _Tp>
+struct is_gencomplex
+    : std::integral_constant<bool,
+                             std::is_same_v<_Tp, complex<double>> ||
+                                 std::is_same_v<_Tp, complex<float>> ||
+                                 std::is_same_v<_Tp, complex<sycl::half>>> {};
+
+////////////////////////////////////////////////////////////////////////////////
+/// DEFINES
+////////////////////////////////////////////////////////////////////////////////
+
+#define _SYCL_EXT_CPLX_INLINE_VISIBILITY                                       \
+  inline __attribute__((__visibility__("hidden"), __always_inline__))
+
+} // namespace experimental
+} // namespace oneapi
+} // namespace ext
+
+} // namespace _V1
+} // namespace sycl