Revert "[c++20] Mark class type NTTPs as done and start defining the feature test macro."

Some of the parts of this work were reverted; stop defining the feature
test macro for now.

This reverts commit b4c63ef.

(cherry picked from commit 564f5b0)

Author: zygoloid

clang/lib/Frontend/InitPreprocessor.cpp

@@ -565,7 +565,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
     Builder.defineMacro("__cpp_aggregate_bases", "201603L");
     Builder.defineMacro("__cpp_structured_bindings", "201606L");
     Builder.defineMacro("__cpp_nontype_template_args",
-                        LangOpts.CPlusPlus20 ? "201911L" : "201411L");
+                        "201411L"); // (not latest)
     Builder.defineMacro("__cpp_fold_expressions", "201603L");
     Builder.defineMacro("__cpp_guaranteed_copy_elision", "201606L");
     Builder.defineMacro("__cpp_nontype_template_parameter_auto", "201606L");

clang/test/Lexer/cxx-features.cpp

@@ -181,7 +181,8 @@
 #error "wrong value for __cpp_structured_bindings"
 #endif
 
-#if check(nontype_template_args, 0, 0, 0, 201411, 201911, 201911)
+#if check(nontype_template_args, 0, 0, 0, 201411, 201411, 201411)
+// FIXME: 201911 in C++20
 #error "wrong value for __cpp_nontype_template_args"
 #endif