Bump the default from gnu++17 to gnu++20.

We should update gnu++2a to gnu++2b too, but not in this CL.

Bug: 311052584
Test: treehugger
Test: m checkbuild
Change-Id: I3637444d74868b3a29b425fd749ecbbe1a45fbf7

cc/cc_test.go

@@ -4818,7 +4818,7 @@ func TestIncludeDirectoryOrdering(t *testing.T) {
 
 	cflags := []string{"-Werror", "-std=candcpp"}
 	cstd := []string{"-std=gnu17", "-std=conly"}
-	cppstd := []string{"-std=gnu++17", "-std=cpp", "-fno-rtti"}
+	cppstd := []string{"-std=gnu++20", "-std=cpp", "-fno-rtti"}
 
 	lastIncludes := []string{
 		"out/soong/ndk/sysroot/usr/include",