noexcept applied to <thread>.

llvm-svn: 160606
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 36101a5b0a89ce6646cc576273551faf6a4548b7
diff --git a/src/thread.cpp b/src/thread.cpp
index f27136a..ffcbe0a 100644
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -52,7 +52,7 @@
 }
 
 unsigned
-thread::hardware_concurrency()
+thread::hardware_concurrency() _NOEXCEPT
 {
 #if defined(CTL_HW) && defined(HW_NCPU)
     unsigned n;