Add #elif test and CWG 3017 context

- Add test coverage for comma operator in #elif directive
- Reference CWG 3017 in comments for additional context
- Addresses reviewer feedback

Author: swote-git

clang/test/Preprocessor/cxx_oper_comma.cpp

@@ -10,7 +10,9 @@
 #if 1, 2
 #endif
 
-// Test 2: Comma in conditional expression
+// Test 2: Comma in conditional expression(CWG3017)
+// Per CWG 3017, this exact case highlights the specification gap
+// where C++ lacks explicit prohibition of comma operators in #if
 // expected-error@+1 {{comma operator in operand of #if}}
 #if 1 ? 1, 0 : 3
 #endif
@@ -24,3 +26,8 @@
 // expected-error@+1 {{expected end of line in preprocessor expression}}
 #if 1, 2, 3
 #endif
+
+// Test 5: Comma in #elif
+#if 0
+#elif (1, 2) // expected-error {{comma operator in operand of #if}}
+#endif