Fix #15040 FN stlcstrConcat with compound assignment (#8857)

Author: chrchr-github

lib/checkstl.cpp

@@ -1986,9 +1986,11 @@ static bool isc_strCall(const Token* tok, const Library::Container* container)
 
 static bool isc_strConcat(const Token* tok)
 {
-    if (!tok->isBinaryOp() || !Token::simpleMatch(tok, "+"))
+    if (!tok->isBinaryOp() || !Token::Match(tok, "+|+="))
         return false;
     for (const Token* op : { tok->astOperand1(), tok->astOperand2() }) { // NOLINT(readability-use-anyofallof)
+        if (tok->isAssignmentOp() && astIsLHS(op))
+            continue;
         const Token* sibling = op->astSibling();
         if (!sibling->valueType())
             continue;

test/teststl.cpp

@@ -5161,6 +5161,12 @@ class TestStl : public TestFixture {
               "}\n");
         ASSERT_EQUALS("[test.cpp:2:17]: (performance) Constructing a std::string from the result of c_str() is slow and redundant. [stlcstrConstructor]\n",
                       errout_str());
+
+        check("void f(std::string& a, const std::string& b) {\n"
+              "    a += b.c_str();\n"
+              "}\n");
+        ASSERT_EQUALS("[test.cpp:2:7]: (performance) Concatenating the result of c_str() and a std::string is slow and redundant. [stlcstrConcat]\n",
+                      errout_str());
     }
 
     void uselessCalls() {