noexcept and constexpr applied to <ios>.

llvm-svn: 160593
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 5d926bf1b8dd15aca2c10733d15ec94dc3086efe
diff --git a/src/ios.cpp b/src/ios.cpp
index 80917a0..732a61b 100644
--- a/src/ios.cpp
+++ b/src/ios.cpp
@@ -401,7 +401,7 @@
 }
 
 void
-ios_base::swap(ios_base& rhs)
+ios_base::swap(ios_base& rhs) _NOEXCEPT
 {
     _VSTD::swap(__fmtflags_, rhs.__fmtflags_);
     _VSTD::swap(__precision_, rhs.__precision_);