C++17: switch the default.

Bug: http://b/111067277
Change-Id: I07af7cd9e65a6e238d7d256c65927fe128a0f052
Test: builds
diff --git a/cc/config/global.go b/cc/config/global.go
index 8fd5914..ef9ac80 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -114,7 +114,7 @@
 	}
 
 	CStdVersion               = "gnu99"
-	CppStdVersion             = "gnu++14"
+	CppStdVersion             = "gnu++17"
 	ExperimentalCStdVersion   = "gnu11"
 	ExperimentalCppStdVersion = "gnu++2a"