[Clang][Sema] Add fortify warnings for stpcpy (#141646)

As mentioned in #142230, I am
adding fortify warnings for functions missing in Clang and I am starting
with stpcpy.

Author: Sharjeel-Khan

clang/lib/Sema/SemaChecking.cpp

@@ -1257,6 +1257,8 @@ void Sema::checkFortifiedBuiltinMemoryFunction(FunctionDecl *FD,
   switch (BuiltinID) {
   default:
     return;
+  case Builtin::BI__builtin_stpcpy:
+  case Builtin::BIstpcpy:
   case Builtin::BI__builtin_strcpy:
   case Builtin::BIstrcpy: {
     DiagID = diag::warn_fortify_strlen_overflow;
@@ -1265,6 +1267,7 @@ void Sema::checkFortifiedBuiltinMemoryFunction(FunctionDecl *FD,
     break;
   }
 
+  case Builtin::BI__builtin___stpcpy_chk:
   case Builtin::BI__builtin___strcpy_chk: {
     DiagID = diag::warn_fortify_strlen_overflow;
     SourceSize = ComputeStrLenArgument(1);

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

@@ -71,6 +71,14 @@ void call_strcpy_nowarn(void) {
   __builtin_strcpy(dst, src);
 }
 
+void call_stpcpy(void) {
+  const char *const src = "abcd";
+  char dst1[5];
+  char dst2[4];
+  __builtin_stpcpy(dst1, src);
+  __builtin_stpcpy(dst2, src); // expected-warning {{'stpcpy' will always overflow; destination buffer has size 4, but the source string has length 5 (including NUL byte)}}
+}
+
 void call_memmove(void) {
   char s1[10], s2[20];
   __builtin_memmove(s2, s1, 20);