Bump default experimental from gnu++2a to gnu++2b.

Bug: 311052584
Test: treehugger
Test: m checkbuild
Change-Id: I5e9d479dd4c14e21fe7d29ea6894f1149fd15e50
diff --git a/cc/config/global.go b/cc/config/global.go
index bc067ad..8b6df30 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -359,7 +359,7 @@
 	CStdVersion               = "gnu17"
 	CppStdVersion             = "gnu++20"
 	ExperimentalCStdVersion   = "gnu2x"
-	ExperimentalCppStdVersion = "gnu++2a"
+	ExperimentalCppStdVersion = "gnu++2b"
 
 	// prebuilts/clang default settings.
 	ClangDefaultBase         = "prebuilts/clang/host"