[Driver][SYCL] Treat stdin as C++ when -fsycl is active (#204968)
1723b7a30145 added a frontend check that rejects C inputs when SYCL mode
is active (since SYCL requires C++). The stdin path in BuildInputs
hardcoded TY_C regardless of driver mode, so `-fsycl -dM -E -` would
pass -x c to cc1 and trigger the new diagnostic.
Fix: use TY_CXX for stdin when IsSYCL.
(cherry picked from commit b37b10d5217b992180ee0a61f343b3edb4cc083e)
