Refer to _LIBCPP_MSVC macro where applicable

Replace preprocess conditions of defined(_MSC_VER) &&
!defined(__clang__) with defined(_LIBCPP_MSVC).  NFC.

Patch by Dave Lee!

llvm-svn: 294171
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: bcc85cbcde661e8bf02a5e0c385f9aa509a8dc39
diff --git a/src/thread.cpp b/src/thread.cpp
index c471b5c..afa8e16 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(_MSC_VER) && ! defined(__clang__)
+#   if defined(_LIBCPP_MSVC)
         _LIBCPP_WARNING("hardware_concurrency not yet implemented")
 #   else
 #       warning hardware_concurrency not yet implemented