Allow trailing comma in assert (#1121)

Author: sookach

regression-tests/pure2-trailing-comma-assert.cpp2

@@ -0,0 +1,3 @@
+main: () = {
+    assert(true, "some_potentially_long_string",);
+}

regression-tests/test-results/apple-clang-14-c++2b/pure2-trailing-comma-assert.cpp.execution

@@ -0,0 +1,24 @@
+
+#define CPP2_IMPORT_STD          Yes
+
+//=== Cpp2 type declarations ====================================================
+
+
+#include "cpp2util.h"
+
+#line 1 "pure2-trailing-comma-assert.cpp2"
+
+
+//=== Cpp2 type definitions and function declarations ===========================
+
+#line 1 "pure2-trailing-comma-assert.cpp2"
+auto main() -> int;
+
+//=== Cpp2 function definitions =================================================
+
+#line 1 "pure2-trailing-comma-assert.cpp2"
+auto main() -> int{
+#line 2 "pure2-trailing-comma-assert.cpp2"
+    if (cpp2::cpp2_default.is_active() && !(true) ) { cpp2::cpp2_default.report_violation(CPP2_CONTRACT_MSG("some_potentially_long_string")); }
+}
+

regression-tests/test-results/apple-clang-14-c++2b/pure2-trailing-comma-assert.cpp.output

@@ -0,0 +1,2 @@
+pure2-trailing-comma-assert.cpp2... ok (all Cpp2, passes safety checks)
+

regression-tests/test-results/apple-clang-15-c++2b/pure2-trailing-comma-assert.cpp.execution

@@ -8006,6 +8006,11 @@ class parser
             }
         }
 
+        //  Consume trailing comma
+        if (curr().type() == lexeme::Comma) {
+            next();
+        }
+
         if (curr().type() != lexeme::RightParen) {
             error("expected ')' at the end of the contract");
             return {};