[Driver] Enable stdlibxx-isystem test on Windows

With `%/t`, we can now compare `-###` output against the installtion dir
without backslash escaping getting in the way. Having to check against
the doubled-up backslashes is ugly, but it's the pattern used in lots of
tests (including for things like debug info where an `-###` alternative
that didn't escape backslashes wouldn't help).

clang/test/Driver/stdlibxx-isystem.cpp

@@ -1,29 +1,28 @@
 // Backslash escaping makes matching against the installation directory fail on
 // Windows. Temporarily disable the test there until we add an option to print
 // the installation directory unescaped.
-// UNSUPPORTED: system-windows
 
 // By default, we should search for libc++ next to the driver.
 // RUN: mkdir -p %t/bin
 // RUN: mkdir -p %t/include/c++/v1
-// RUN: %clang -target aarch64-linux-gnu -ccc-install-dir %t/bin \
+// RUN: %clang -target aarch64-linux-gnu -ccc-install-dir %/t/bin \
 // RUN:   -stdlib=libc++ -fsyntax-only %s -### 2>&1 | \
 // RUN:   FileCheck -check-prefix=LIBCXX %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %t/bin \
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %/t/bin \
 // RUN:   -stdlib=libc++ -fsyntax-only %s -### 2>&1 | \
 // RUN:   FileCheck -check-prefix=LIBCXX %s
 // LIBCXX: InstalledDir: [[INSTALLDIR:.+$]]
-// LIBCXX: "-internal-isystem" "[[INSTALLDIR]]/../include/c++/v1"
+// LIBCXX: "-internal-isystem" "[[INSTALLDIR]]{{/|\\\\}}..{{/|\\\\}}include{{/|\\\\}}c++{{/|\\\\}}v1"
 
 // Passing -stdlib++-isystem should suppress the default search.
-// RUN: %clang -target aarch64-linux-gnu -ccc-install-dir %t/bin \
+// RUN: %clang -target aarch64-linux-gnu -ccc-install-dir %/t/bin \
 // RUN:   -stdlib++-isystem /tmp/foo -stdlib++-isystem /tmp/bar -stdlib=libc++ \
 // RUN:   -fsyntax-only %s -### 2>&1 | FileCheck -check-prefix=NODEFAULT %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %t/bin \
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %/t/bin \
 // RUN:   -stdlib++-isystem /tmp/foo -stdlib++-isystem /tmp/bar -stdlib=libc++ \
 // RUN:   -fsyntax-only %s -### 2>&1 | FileCheck -check-prefix=NODEFAULT %s
 // NODEFAULT: InstalledDir: [[INSTALLDIR:.+$]]
-// NODEFAULT-NOT: "-internal-isystem" "[[INSTALLDIR]]/../include/c++/v1"
+// NODEFAULT-NOT: "-internal-isystem" "[[INSTALLDIR]]{{/|\\\\}}..{{/|\\\\}}include{{/|\\\\}}c++{{/|\\\\}}v1"
 
 // And we should add it as an -internal-isystem.
 // RUN: %clang -target aarch64-linux-gnu -ccc-install-dir %t/bin \