[SYCL] Implement "swizzle" member function on swizzles (#16353)

It brings it closer to SYCL spec and makes more consistent by having
"named" swizzle member functions (like `.hi()`/`.lo()`) and `.swizzle<...>()`
behave in a similar way, i.e. we still have a bug when doing a swizzle on an
expression tree.

Note that the whole "expression tree" machinery is not
standard-conformant and will be completely removed separately (under
`-fpreview-breaking-changes` flag).

I still want to implement this partial bugfix so that I could switch to
a unified mixin-based implementation for swizzles on `vec`/`swizzle`
classes, instead of doing preprocessor tricks with `swizzles.def` file.

sycl/include/sycl/vector.hpp

@@ -1207,6 +1207,16 @@ class SwizzleOp {
   };
 
 public:
+  template <int... swizzleIndexes>
+  ConstSwizzle<Indexer<swizzleIndexes>::value...> swizzle() const {
+    return m_Vector;
+  }
+
+  template <int... swizzleIndexes>
+  Swizzle<Indexer<swizzleIndexes>::value...> swizzle() {
+    return m_Vector;
+  }
+
 #ifdef __SYCL_ACCESS_RETURN
 #error "Undefine __SYCL_ACCESS_RETURN macro"
 #endif

sycl/test/basic_tests/vectors/swizzle.cpp

@@ -1,9 +1,6 @@
 // RUN: %clangxx -fsycl %s -o %t_default.out
 // RUN: %t_default.out
 
-// FIXME: Everything should compile cleanly.
-// RUN: %clangxx -fsycl -fsycl-device-only -DCHECK_ERRORS -fsyntax-only -Xclang -verify -Xclang -verify-ignore-unexpected=note,error %s
-
 #include <sycl/vector.hpp>
 
 int main() {
@@ -15,27 +12,22 @@ int main() {
   // FIXME: Should be "4":
   assert((sw + sw).lo()[0] == 2);
 
-  // FIXME: The below should compile.
-#if CHECK_ERRORS
-  // expected-error-re@+1 {{no template named 'swizzle' in {{.*}}}}
   assert(sw.swizzle<0>()[0] == 2);
-  // expected-error-re@+1 {{no template named 'swizzle' in {{.*}}}}
   assert(sw.swizzle<1>()[0] == 3);
 
   {
-    // expected-error-re@+1 {{no template named 'swizzle' in {{.*}}}}
     auto tmp = sw.swizzle<1, 0>();
     assert(tmp[0] == 3);
     assert(tmp[1] == 2);
   }
 
   {
-    // expected-error-re@+1 {{no template named 'swizzle' in {{.*}}}}
     auto tmp = (sw + sw).swizzle<1, 0>();
-    assert(tmp[0] == 6);
-    assert(tmp[1] == 4);
+
+    // FIXME: Should be "6" and "4", respectively.
+    assert(tmp[0] == 3);
+    assert(tmp[1] == 2);
   }
-#endif
 
   return 0;
 }