[SYCL][NFC] Re-enable sycl.c test on Windows (#15433)

The issue dealt with the resetting of the PATH environment variable.
This was masking potential runtime dependencies (DLLs and such) for
other tools being used when running the test. The solution is to append
the PATH for the dummy `aoc` instead of overwriting.

Author: mdtoguchi

clang/test/Driver/sycl.c

@@ -104,7 +104,7 @@
 // Test with a bad argument is expected to fail
 // RUN: not %clang -fsycl-help=foo %s 2>&1 | FileCheck %s --check-prefix=SYCL-HELP-BADARG
 // RUN: %clang -### -fsycl-help=gen %s 2>&1 | FileCheck %s --check-prefix=SYCL-HELP-GEN
-// RUN: env PATH=%t-sycl-dir %clang -### -fsycl-help=fpga %s 2>&1 | FileCheck %s --check-prefixes=SYCL-HELP-FPGA,SYCL-HELP-FPGA-OUT -DDIR=%t-sycl-dir
+// RUN: env "PATH=%t-sycl-dir%{pathsep}%PATH%" %clang -### -fsycl-help=fpga %s 2>&1 | FileCheck %s --check-prefixes=SYCL-HELP-FPGA,SYCL-HELP-FPGA-OUT -DDIR=%t-sycl-dir
 // RUN: %clang -### -fsycl-help=x86_64 %s 2>&1 | FileCheck %s --check-prefix=SYCL-HELP-CPU
 // RUN: %clang -### -fsycl-help %s 2>&1 | FileCheck %s --check-prefixes=SYCL-HELP-GEN,SYCL-HELP-FPGA,SYCL-HELP-CPU
 // SYCL-HELP-BADARG: unsupported argument 'foo' to option '-fsycl-help='