[SYCL][Test] Track amount of inter-dependencies between headers

To improve compile-time, we may one day split `sycl/sycl.hpp` into
separate headers, but we need to make sure that those headers don't
include every other header so that there is an actual benefit of using
them.

This PR introduced a test to track that certain headers which we already
consider isolated are not used from anywhere else except from
`sycl/sycl.hpp`.

Note that for now we allow extension headers to include everything and
only focusing on the core SYCL headers.

Author: AlexeySachkov

sycl/test/sycl-headers-are-isolated.cpp

@@ -0,0 +1,27 @@
+// sycl/sycl.hpp is a very heavy header, because it includes all SYCL feature.
+//
+// In order to be able to split it, we need to make sure that all
+// inter-dependencies between headers are resolved. We already have a set of
+// tests which make sure that every header can be compiled if included alone,
+// but to achieve that, every header can simply include every other header.
+//
+// Considering that the main reason for sycl.hpp splitting is compilation time
+// improvement, it is important that SYCL headers do not include each other
+// unnecessary to make sure that when you include a single specific header, you
+// only get specific functionality and nothing extra.
+//
+// Note that this test ignores "ext" subfolder for now and concentrates on
+// core SYCL functionality for now.
+//
+// REQUIRES: linux
+//
+// RUN: grep -rl "#include <sycl/sub_group.hpp>" %sycl_include/sycl \
+// RUN:     --exclude-dir=*ext* | FileCheck %s --check-prefix=SUB-GROUP
+// SUB-GROUP: sycl/sycl.hpp
+// SUB-GROUP-EMPTY:
+//
+// RUN: grep -rl "#include <sycl/stream.hpp>" %sycl_include/sycl \
+// RUN:     --exclude-dir=*ext* | FileCheck %s --check-prefix=STREAM
+// STREAM: sycl/sycl.hpp
+// STREAM-EMPTY:
+