Internal coverage improvements. No functional change.

src/V3LanguageWords.h

@@ -48,7 +48,7 @@ public:
 
 private:
     static Singleton& s() {
-        static Singleton s_s;
+        static Singleton s_s;  // LCOV_EXCL_BR_LINE
         return s_s;
     }
 };

src/V3PreLex.l

@@ -595,11 +595,11 @@ string V3PreLex::cleanDbgStrg(const string& in) {
 }
 
 void V3PreLex::unused() {
-    if (0) {
+    if (VL_UNCOVERABLE(false)) {  // LCOV_EXCL_START
         // Prevent unused warnings
         yy_top_state();
         yyerror((char*)"");
-    }
+    }  // LCOV_EXCL_STOP
 }
 
 /*###################################################################

src/Verilator.cpp

@@ -549,9 +549,9 @@ static void verilate(const string& argString) {
     }
     // Undocumented debugging - cannot be a switch as then command line
     // would mismatch forcing non-identicalness when we set it
-    if (!V3Os::getenvStr("VERILATOR_DEBUG_SKIP_IDENTICAL", "").empty()) {
+    if (!V3Os::getenvStr("VERILATOR_DEBUG_SKIP_IDENTICAL", "").empty()) {  // LCOV_EXCL_START
         v3fatalSrc("VERILATOR_DEBUG_SKIP_IDENTICAL w/ --skip-identical: Changes found\n");
-    }
+    }  // LCOV_EXCL_STOP
 
     //--FRONTEND------------------