[libc++] Run substitutes-in-compile-flags.sh.cpp test on Windows.

Fix for substitutes-in-compile-flags.sh.cpp to run it properly on Windows platform.

Differential Revision: https://reviews.llvm.org/D102048

Author: vvereschaka

libcxx/test/libcxx/selftest/additional_compile_flags/substitutes-in-compile-flags.sh.cpp

@@ -7,12 +7,12 @@
 //===----------------------------------------------------------------------===//
 
 // This test greps for %t, which is expanded to a path with backslashes. When
-// that is passed to grep, those backslashes would have to be escaped, which we
-// don't do right now.
-// UNSUPPORTED: windows
+// that is passed to grep, those backslashes must be escaped. We escape those
+// within the pattern into a file and use this file with 'grep'.
 
 // Make sure that substitutions are performed inside additional compiler flags.
 
 // ADDITIONAL_COMPILE_FLAGS: -I %t.1
 // ADDITIONAL_COMPILE_FLAGS: -isystem %t.2 , -isysroot %t.3
-// RUN: echo "%{compile_flags}" | grep -e '-I %t.1 -isystem %t.2 -isysroot %t.3'
+// RUN: echo "-I %t.1 -isystem %t.2 -isysroot %t.3" | sed "s/\\\/\\\\\\\/g" > %t.escaped.grep
+// RUN: echo "%{compile_flags}" | grep -e -f %t.escaped.grep