[clang][test] Add test for comma operator rejection in preprocessor conditionals (llvm#155570)

Add test coverage to ensure comma operators remain properly rejected in
`#if` directives.

Per CWG 1436, comma is not among the permitted operators in preprocessor
conditional expressions. This test prevents regressions and clarifies
the intended behavior.

Fixes llvm#132822

Author: swote-git

clang/test/Preprocessor/cxx_oper_comma.cpp

@@ -0,0 +1,45 @@
+// RUN: %clang_cc1 -E -pedantic-errors %s -verify -std=c++98
+// RUN: %clang_cc1 -E -pedantic-errors %s -verify -std=c++11
+// RUN: %clang_cc1 -E -pedantic-errors %s -verify -std=c++14
+// RUN: %clang_cc1 -E -pedantic-errors %s -verify -std=c++17
+// RUN: %clang_cc1 -E -pedantic-errors %s -verify -std=c++20
+// RUN: %clang_cc1 -E -pedantic-errors %s -verify=expected,since-cxx23 -std=c++23 -Wno-c23-extensions
+// RUN: %clang_cc1 -E -pedantic-errors %s -verify=expected,since-cxx23 -std=c++2c -Wno-c23-extensions
+
+// Test 1: Top-level comma
+// expected-error@+1 {{expected end of line in preprocessor expression}}
+#if 1, 2
+#endif
+
+// 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
+
+// Test 3: Parenthesized comma
+// expected-error@+1 {{comma operator in operand of #if}}
+#if (1, 2)
+#endif
+
+// Test 4: Multiple commas
+// 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
+
+// Test 6: Leading comma (syntax error)
+// expected-error@+1 {{invalid token at start of a preprocessor expression}}
+#if ,
+#endif
+
+// Test 7: Comma in #embed limit parameter (C++23+)
+#if __cplusplus >= 202302L
+// since-cxx23-error@+1 {{expected ')'}}
+#embed "jk.txt" limit(1, 2)
+#endif