C++17: Support "if constexpr" (by simplifying it to plain if() statement)

This might lead to complaints about constant expressions as if() statement, but should fix syntax errors.

lib/tokenize.cpp

@@ -3532,6 +3532,8 @@ bool Tokenizer::simplifyTokenList1(const char FileName[])
                 // 'for each ( )' -> 'asm ( )'
                 tok->str("asm");
                 tok->deleteNext();
+            } else if (tok->strAt(1) == "constexpr") {
+                tok->deleteNext();
             } else {
                 syntaxError(tok);
             }

test/testtokenize.cpp

@@ -82,6 +82,7 @@ private:
         TEST_CASE(syntax_case_default);
 
         TEST_CASE(foreach);     // #3690
+        TEST_CASE(ifconstexpr);
 
         TEST_CASE(combineOperators);
 
@@ -905,6 +906,10 @@ private:
         ASSERT_EQUALS("void f ( ) { asm ( \"char c in MyString\" ) { Console :: Write ( c ) ; } }", tokenizeAndStringify(code));
     }
 
+    void ifconstexpr() {
+        ASSERT_EQUALS("void f ( ) { if ( FOO ) { bar ( c ) ; } }", tokenizeAndStringify("void f() { if constexpr ( FOO ) { bar(c); } }"));
+    }
+
     void combineOperators() {
         ASSERT_EQUALS("; private: ;", tokenizeAndStringify(";private:;", false));
         ASSERT_EQUALS("; protected: ;", tokenizeAndStringify(";protected:;", false));