[Clang][Sema] Add fortify warnings for strcat (llvm#168965)

Continue to add fortify warnings that are missing in Clang for string
functions as part of llvm#142230

Author: venk-ks

clang/lib/Sema/SemaChecking.cpp

@@ -1263,6 +1263,8 @@ void Sema::checkFortifiedBuiltinMemoryFunction(FunctionDecl *FD,
   switch (BuiltinID) {
   default:
     return;
+  case Builtin::BI__builtin_strcat:
+  case Builtin::BIstrcat:
   case Builtin::BI__builtin_stpcpy:
   case Builtin::BIstpcpy:
   case Builtin::BI__builtin_strcpy:
@@ -1273,6 +1275,7 @@ void Sema::checkFortifiedBuiltinMemoryFunction(FunctionDecl *FD,
     break;
   }
 
+  case Builtin::BI__builtin___strcat_chk:
   case Builtin::BI__builtin___stpcpy_chk:
   case Builtin::BI__builtin___strcpy_chk: {
     DiagID = diag::warn_fortify_strlen_overflow;

clang/test/Sema/warn-fortify-source.c

@@ -76,6 +76,14 @@ void call_strcpy_nowarn(void) {
   __builtin_strcpy(dst, src);
 }
 
+void call_strcat(void) {
+  const char *const src = "abcd";
+  char dst1[5];
+  char dst2[4];
+  __builtin_strcat(dst1, src);
+  __builtin_strcat(dst2, src); // expected-warning {{'strcat' will always overflow; destination buffer has size 4, but the source string has length 5 (including NUL byte)}}
+}
+
 void call_stpcpy(void) {
   const char *const src = "abcd";
   char dst1[5];