Revert "Suppress non-conforming GNU paste extension in all standard-conforming modes"

This reverts commit f453793.
This reverts commit b43c26d.

This GNU and MSVC extension turns out to be very popular. Most projects
are not using C++20, so cannot use the new __VA_OPT__ feature to be
standards conformant. The other workaround, using -std=gnu*, enables too
many language extensions and isn't viable.

Until there is a way for users to get the behavior provided by the
`, ## __VA_ARGS__` extension in the -std=c++17 and earlier language
modes, we need to revert this.

(cherry picked from commit 61a66e4)

Author: rnk

clang/lib/Lex/TokenLexer.cpp

@@ -148,12 +148,12 @@ bool TokenLexer::MaybeRemoveCommaBeforeVaArgs(
     return false;
 
   // GCC removes the comma in the expansion of " ... , ## __VA_ARGS__ " if
-  // __VA_ARGS__ is empty, but not in strict mode where there are no
-  // named arguments, where it remains.  With GNU extensions, it is removed
-  // regardless of named arguments.
+  // __VA_ARGS__ is empty, but not in strict C99 mode where there are no
+  // named arguments, where it remains.  In all other modes, including C99
+  // with GNU extensions, it is removed regardless of named arguments.
   // Microsoft also appears to support this extension, unofficially.
-  if (!PP.getLangOpts().GNUMode && !PP.getLangOpts().MSVCCompat &&
-      Macro->getNumParams() < 2)
+  if (PP.getLangOpts().C99 && !PP.getLangOpts().GNUMode
+        && Macro->getNumParams() < 2)
     return false;
 
   // Is a comma available to be removed?

clang/test/Preprocessor/macro_fn_comma_swallow2.c

@@ -1,16 +1,11 @@
 // Test the __VA_ARGS__ comma swallowing extensions of various compiler dialects.
 
 // RUN: %clang_cc1 -E %s | FileCheck -check-prefix=GCC -strict-whitespace %s
-// RUN: %clang_cc1 -E -std=c90 %s | FileCheck -check-prefix=C99 -strict-whitespace %s
 // RUN: %clang_cc1 -E -std=c99 %s | FileCheck -check-prefix=C99 -strict-whitespace %s
 // RUN: %clang_cc1 -E -std=c11 %s | FileCheck -check-prefix=C99 -strict-whitespace %s
 // RUN: %clang_cc1 -E -x c++ %s | FileCheck -check-prefix=GCC -strict-whitespace %s
-// RUN: %clang_cc1 -E -x c++ -std=c++03 %s | FileCheck -check-prefix=C99 -strict-whitespace %s
-// RUN: %clang_cc1 -E -x c++ -std=c++11 %s | FileCheck -check-prefix=C99 -strict-whitespace %s
 // RUN: %clang_cc1 -E -std=gnu99 %s | FileCheck -check-prefix=GCC -strict-whitespace %s
 // RUN: %clang_cc1 -E -fms-compatibility %s | FileCheck -check-prefix=MS -strict-whitespace %s
-// RUN: %clang_cc1 -E -x c++ -fms-compatibility %s | FileCheck -check-prefix=MS -strict-whitespace %s
-// RUN: %clang_cc1 -E -x c++ -std=c++11 -fms-compatibility %s | FileCheck -check-prefix=MS -strict-whitespace %s
 // RUN: %clang_cc1 -E -DNAMED %s | FileCheck -check-prefix=GCC -strict-whitespace %s
 // RUN: %clang_cc1 -E -std=c99 -DNAMED %s | FileCheck -check-prefix=C99 -strict-whitespace %s