[clang] [test] Skip a test that sets PATH= on Windows

The same has been done in a couple other existing tests, that also
are skipped on Windows (e.g. ld-path.c). Some tests that really
do want to test setting the path on Windows does it differently,
see e.g. ps4-ps5-linker-win.c.

Since a65771f, the spirv-toolchain.cl
test does one test where PATH is set. Setting PATH does work in
some build configurations - however, if built with e.g. llvm-mingw,
the built Clang executable depends on libc++.dll (and libunwind.dll)
which are found in PATH. If the PATH is overridden, the newly built
Clang executable no longer can run.

Author: mstorsjo

clang/test/Driver/spirv-toolchain.cl

@@ -1,3 +1,8 @@
+/// One test uses the PATH environment variable; on Windows, we may need to retain
+/// the original path for the built Clang binary to be able to execute (as it is
+/// used for locating dependent DLLs).
+// UNSUPPORTED: system-windows
+
 // Check object emission.
 // RUN: %clang -### --target=spirv64 -x cl -c %s 2>&1 | FileCheck --check-prefix=SPV64 %s
 // RUN: %clang -### --target=spirv64 %s 2>&1 | FileCheck --check-prefix=SPV64 %s