Bump "experimental" C++ to C++2a.

C++17 is so last year.

Test: treehugger
Change-Id: Ifd5e6d4a4837fe037837c49c31ee2fe1499e2d79
diff --git a/cc/config/global.go b/cc/config/global.go
index e2377e3..8fd5914 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -116,7 +116,7 @@
 	CStdVersion               = "gnu99"
 	CppStdVersion             = "gnu++14"
 	ExperimentalCStdVersion   = "gnu11"
-	ExperimentalCppStdVersion = "gnu++1z"
+	ExperimentalCppStdVersion = "gnu++2a"
 
 	NdkMaxPrebuiltVersionInt = 27