[SYCL] Cleanup the deprecated Intel attribute logic (#3036)

We previously sprinkled deprecated attribute logic around to a few places
when we could instead use a single helper function to do the work for us.

This also adds a fix-it hint when we find a deprecated attribute to help
the user convert to the new spelling.

Author: AaronBallman

clang/include/clang/Sema/Sema.h

@@ -12884,6 +12884,11 @@ class Sema final {
   void MarkDevice();
   void MarkSyclSimd();
 
+  /// Diagnoses an attribute in the 'intelfpga' namespace and suggests using
+  /// the attribute in the 'intel' namespace instead.
+  void CheckDeprecatedSYCLAttributeSpelling(const ParsedAttr &A,
+                                            StringRef NewName = "");
+
   /// Creates a SemaDiagnosticBuilder that emits the diagnostic if the current
   /// context is "used as device code".
   ///