[Tests] Improve tests about warning-free SYCL headers

[AlexeySachkov]

Is your feature request related to a problem? Please describe

This is prompted by discussion we had in #15064: we already have some testing to check that SYCL headers are warning-free, but it can always be improved. In particular, we allow to use a 3rd party compiler to build host part of SYCL applications and in that phase/mode SYCL headers are not considered to be system headers, so any warnings coming out of them may break -Werror builds or just annoy users polluting their build logs.

Describe the solution you would like

As a first step, we should copy existing warnings.cpp test and compile it using gcc instead of clang++:

llvm/sycl/test/warnings/warnings.cpp

Lines 1 to 3 in b8615ff

	// RUN: %clangxx -fsycl --no-system-header-prefix=sycl -fsyntax-only -Wall -Wextra -Werror -Wno-ignored-attributes -Wno-deprecated-declarations -Wpessimizing-move -Wmismatched-tags -Wno-unknown-cuda-version -Wno-unused-command-line-argument %s
	// RUN: %clangxx -fsycl -E --no-system-header-prefix=sycl %s -o %t.ii
	// RUN: %clangxx -fsycl -fsyntax-only -Wall -Wextra -Werror -Wno-ignored-attributes -Wno-deprecated-declarations -Wpessimizing-move -Wmismatched-tags -Wno-unknown-cuda-version -Wno-unused-command-line-argument %t.ii

Going forward, this (and existing) tests could be further expanded, but having a test with gcc would be already a good start.

Describe alternatives you have considered

No response

Additional context

No response