[clang][SYCL] Disable float128 device mode diagnostic (#128513) (#17430)

Cherry-pick of: llvm/llvm-project#128513

Fixes: #16903

------
This diagnostic is disabled for device compilation as float128 is not
supported on the device side.

Other diagnostics are already covering the cases where float128 is
actually used in the kernel code, and it's already tested for in the
existing test.

This is expanding on the patch 318bff6 that handled this for cuda
compilation.

Author: npmiller

clang/lib/Sema/SemaDeclAttr.cpp

@@ -4725,7 +4725,8 @@ void Sema::AddModeAttr(Decl *D, const AttributeCommonInfo &CI,
 
   if (NewElemTy.isNull()) {
     // Only emit diagnostic on host for 128-bit mode attribute
-    if (!(DestWidth == 128 && getLangOpts().CUDAIsDevice))
+    if (!(DestWidth == 128 &&
+          (getLangOpts().CUDAIsDevice || getLangOpts().SYCLIsDevice)))
       Diag(AttrLoc, diag::err_machine_mode) << 1 /*Unsupported*/ << Name;
     return;
   }

clang/test/SemaSYCL/float128.cpp

@@ -5,6 +5,7 @@
 
 sycl::queue deviceQueue;
 
+typedef _Complex float __cfloat128 __attribute__ ((__mode__ (__TC__)));
 typedef __float128 BIGTY;
 
 template <class T>