[SYCL] Avoid testing race condition

Use no output file instead of default output file in LIT tests.
This occasionally caused errors on windows when multiple files compile
to the same output file. Using `-fsyntax-only` hjere also avoids running the full compiler pipeline

See #7872 for more context

Author: ayylol

sycl/test/basic_tests/header_footer_diagnostics.cpp

@@ -1,4 +1,4 @@
-// RUN: %clangxx -fsycl -Werror=reserved-identifier -Werror=old-style-cast %s
+// RUN: %clangxx -fsycl -Werror=reserved-identifier -Werror=old-style-cast %s -fsyntax-only
 
 // Check that the generated header and footer files do not generate
 // errors when pedantic warnings are enabled.

sycl/test/basic_tests/is_group_trait.cpp

@@ -1,4 +1,4 @@
-// RUN: %clangxx -fsycl %s
+// RUN: %clangxx -fsycl %s -fsyntax-only
 
 #include <sycl/sycl.hpp>
 

sycl/test/esimd/slm_init_local_accessor.cpp

@@ -1,4 +1,4 @@
-// RUN: %clangxx -fsycl  %s
+// RUN: %clangxx -fsycl  %s -fsyntax-only
 
 // This test verifies usage of slm_init and local_accessor in different kernels
 // passes.