[SYCL][NFC] Enable more warnings for SemaSYCL (#19259)

There are few compiler flags which are required in accordance with our
internal guidelines, but we can't apply them globally, because the
upstream code is not warning-free.

However, we do have two isolated downstream files: `SemaSYCL.cpp` and
`SemaSYCLDeclAttr.cpp` where we can apply those extra flags without
concerns that some code will come to us from the upstream.

Absolute most of the changes is fixing `-Wunused-parameter` warnings

Author: AlexeySachkov

clang/lib/Sema/CMakeLists.txt

@@ -12,6 +12,40 @@ if (MSVC)
   set_source_files_properties(SemaSYCL.cpp PROPERTIES COMPILE_FLAGS /bigobj)
 endif()
 
+if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+  set(is_clang TRUE)
+endif()
+if(CMAKE_CXX_COMPILER_ID MATCHES "GNU")
+  set(is_gcc TRUE)
+endif()
+if(CMAKE_CXX_COMPILER_ID MATCHES "IntelLLVM")
+  set(is_icpx TRUE)
+endif()
+if(CMAKE_CXX_COMPILER_ID MATCHES "MSVC")
+  set(is_msvc TRUE)
+endif()
+
+set(linux_opts "-Wall;-Wextra")
+if (CMAKE_BUILD_TYPE MATCHES "Release")
+  list(APPEND linux_opts "-Wconversion;-Wimplicit-fallthrough")
+endif()
+
+if (is_gcc OR is_clang OR (is_icpx AND NOT WIN32))
+  set(opts_to_apply ${linux_opts})
+elseif (is_msvc)
+  set(opts_to_apply "/W4")
+elseif (is_icpx and WIN32)
+  set(opts_to_apply "/Wall")
+endif()
+
+if (opts_to_apply)
+  set_source_files_properties(
+    SemaSYCL.cpp SemaSYCLDeclAttr.cpp
+    PROPERTIES
+      COMPILE_OPTIONS "${opts_to_apply}"
+  )
+endif()
+
 clang_tablegen(OpenCLBuiltins.inc -gen-clang-opencl-builtins
   SOURCE OpenCLBuiltins.td
   TARGET ClangOpenCLBuiltinsImpl