Remove usages of _LIBCPP_MSVC which is never defined

llvm-svn: 302736
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 0c6e7ae4cc8039af611d7a5cf153d9bb7664f3e2
diff --git a/src/thread.cpp b/src/thread.cpp
index 3f283c3..412a8fa 100644
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -99,7 +99,7 @@
 #else  // defined(CTL_HW) && defined(HW_NCPU)
     // TODO: grovel through /proc or check cpuid on x86 and similar
     // instructions on other architectures.
-#   if defined(_LIBCPP_MSVC)
+#   if defined(_LIBCPP_WARNING)
         _LIBCPP_WARNING("hardware_concurrency not yet implemented")
 #   else
 #       warning hardware_concurrency not yet implemented