Fix build with libc++ (#19521)

We previously added support for building `libsycl` individually with
`libc++`. At that time, `libsycl` didn't link against `libLLVM` so they
did not need to be using the same STL implementation.

Now, `libsycl` does link against `libLLVM`, one example
[here](https://github.com/intel/llvm/blob/sycl/sycl/source/CMakeLists.txt#L98).

Remove all the code that was to support `libsycl` in particular and just
make it work with the normal option LLVM uses, `LLVM_ENABLE_LIBCXX`.
Most of the code I added actually checks `LLVM_LIBCXX_USED`, because
LLVM checks at configure time if `libc++` actually works even when
passed `LLVM_ENABLE_LIBCXX`, and if it fails, it falls back to
`libstdc++`, and most of the time we actually want to know if we are
actually using `libc++`.

I added a CMake option `SYCL_LIBDEVICE_CXX_FLAGS` to set the flags for
libdevice because it will be required if someone doesn't use the LLVM
option and just does it through `CMAKE_CXX_FLAGS`, as those aren't used
for libdevice but it still uses the STL.

The two changes in
`unified-runtime/source/loader/layers/sanitizer/msan/msan_libdevice.hpp`
and `sycl/source/builtins/common_functions.cpp` fix compile errors with
`libc++`.

I also plan to add a job to the nightly to lock this down.

intel/llvm#19347

---------

Signed-off-by: Sarnie, Nick <nick.sarnie@intel.com>

Author: sarnex

source/loader/CMakeLists.txt

@@ -211,21 +211,10 @@ if(UR_ENABLE_SANITIZER)
         # In in-tree build, if LLVM is built with libc++, we also need to build
         # symbolizer.cpp with libc++ abi and link libc++ in.
         if(NOT UR_STANDALONE_BUILD AND LLVM_LIBCXX_USED)
-            execute_process(
-                COMMAND ${CMAKE_CXX_COMPILER} --print-file-name=libc++.a
-                OUTPUT_VARIABLE LIBCXX_PATH
-                OUTPUT_STRIP_TRAILING_WHITESPACE)
-            execute_process(
-                COMMAND ${CMAKE_CXX_COMPILER} --print-file-name=libc++abi.a
-                OUTPUT_VARIABLE LIBCXX_ABI_PATH
-                OUTPUT_STRIP_TRAILING_WHITESPACE)
             set_property(SOURCE
                 ${symbolizer_sources}
                 APPEND_STRING PROPERTY COMPILE_FLAGS
                 " -stdlib=libc++ ")
-            if(NOT EXISTS ${LIBCXX_PATH} OR NOT EXISTS ${LIBCXX_ABI_PATH})
-                message(FATAL_ERROR "libc++ is required but can't find the libraries")
-            endif()
             # Link with gcc_s fisrt to avoid some symbols resolve to libc++/libc++abi/libunwind's one
             target_link_libraries(ur_loader PRIVATE gcc_s ${LIBCXX_PATH} ${LIBCXX_ABI_PATH})
         endif()

source/loader/layers/sanitizer/msan/msan_libdevice.hpp

@@ -14,6 +14,7 @@
 #pragma once
 
 #include "sanitizer_common/sanitizer_libdevice.hpp"
+#include <cstddef>
 
 #if !defined(__SPIR__) && !defined(__SPIRV__)
 namespace ur_sanitizer_layer {