Mark private function __sign as constexpr.

llvm-svn: 360167
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 51497fb8b379878cac0f122b44344a0a3b6a294f
diff --git a/include/numeric b/include/numeric
index 6be6080..4b08611 100644
--- a/include/numeric
+++ b/include/numeric
@@ -556,7 +556,7 @@
 
 
 template <typename _Tp>
-int __sign(_Tp __val) {
+constexpr int __sign(_Tp __val) {
     return (_Tp(0) < __val) - (__val < _Tp(0));
 }