C++23 preprocessor extensions

- [#elifdef and #elifndef](https://en.cppreference.com/w/cpp/preprocessor/conditional) [P2334R1](https://wg21.link/P2334R1)
- [#warning](https://en.cppreference.com/w/cpp/preprocessor/error) [P2437R1](https://wg21.link/P2437R1)
- linked to SonarOpenCommunity#2536

cxx-squid/src/test/java/org/sonar/cxx/preprocessor/PPGrammarImplTest.java

@@ -332,6 +332,7 @@ void warningLine() {
     p.setRootRule(g.rule(PPGrammarImpl.warningLine));
 
     assertThat(p).matches("#warning foo");
+    assertThat(p).matches("#warning");
   }
 
   @Test

cxx-squid/src/test/resources/parser/own/C++23/elifxdef.cc

@@ -0,0 +1,48 @@
+void sample() {
+
+// Note that if a compiler does not support C++23's #elifdef/#elifndef
+// directives then the "unexpected" block (see below) will be selected.
+#ifdef CPU
+    std::cout << "4: no1\n";
+#elifdef GPU
+    std::cout << "4: no2\n";
+#elifndef RAM
+    std::cout << "4: yes\n"; // expected block
+#else
+    std::cout << "4: no!\n"; // unexpectedly selects this block by skipping
+                             // unknown directives and "jumping" directly
+                             // from "#ifdef CPU" to this "#else" block
+#endif
+
+// To fix the problem above we may conditionally define the
+// macro ELIFDEF_SUPPORTED only if the C++23 directives
+// #elifdef/#elifndef are supported.
+#if 0
+#elifndef UNDEFINED_MACRO
+#define ELIFDEF_SUPPORTED
+#else
+#endif
+
+#ifdef ELIFDEF_SUPPORTED
+    #ifdef CPU
+        std::cout << "4: no1\n";
+    #elifdef GPU
+        std::cout << "4: no2\n";
+    #elifndef RAM
+        std::cout << "4: yes\n"; // expected block
+    #else
+        std::cout << "4: no3\n";
+    #endif
+#else // when #elifdef unsupported use old verbose `#elif defined`
+    #ifdef CPU
+        std::cout << "4: no1\n";
+    #elif defined GPU
+        std::cout << "4: no2\n";
+    #elif !defined RAM
+        std::cout << "4: yes\n"; // expected block
+    #else
+        std::cout << "4: no3\n";
+    #endif
+#endif
+
+}

cxx-squid/src/test/resources/parser/own/C++23/warning.cc

@@ -0,0 +1,4 @@
+#error
+#error "diagnostic-message"
+#warning
+#warning "diagnostic-message (since C++23)"