Add __warn_memset_zero_len builtin as a workaround for glibc issue

Glibc issue: https://sourceware.org/bugzilla/show_bug.cgi?id=25399
The fix consist in considering the missing function as a builtin lowered to a nop.

Differential Revision: https://reviews.llvm.org/D72869

(cherry picked from commit d293417)

clang/include/clang/Basic/Builtins.def

@@ -788,6 +788,9 @@ BUILTIN(__builtin_abort, "v", "Fnr")
 BUILTIN(__builtin_index, "c*cC*i", "Fn")
 BUILTIN(__builtin_rindex, "c*cC*i", "Fn")
 
+// ignored glibc builtin, see https://sourceware.org/bugzilla/show_bug.cgi?id=25399
+BUILTIN(__warn_memset_zero_len, "v", "nU")
+
 // Microsoft builtins.  These are only active with -fms-extensions.
 LANGBUILTIN(_alloca,          "v*z", "n", ALL_MS_LANGUAGES)
 LANGBUILTIN(__annotation,     "wC*.","n", ALL_MS_LANGUAGES)

clang/lib/CodeGen/CGBuiltin.cpp

@@ -3222,6 +3222,8 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
         Builder.CreateZExt(EmitSignBit(*this, EmitScalarExpr(E->getArg(0))),
                            ConvertType(E->getType())));
   }
+  case Builtin::BI__warn_memset_zero_len:
+    return RValue::getIgnored();
   case Builtin::BI__annotation: {
     // Re-encode each wide string to UTF8 and make an MDString.
     SmallVector<Metadata *, 1> Strings;

clang/test/CodeGen/builtins.c

@@ -453,6 +453,13 @@ void test_builtin_launder(int *p) {
   int *d = __builtin_launder(p);
 }
 
+// __warn_memset_zero_len should be NOP, see https://sourceware.org/bugzilla/show_bug.cgi?id=25399
+// CHECK-LABEL: define void @test___warn_memset_zero_len
+void test___warn_memset_zero_len() {
+  // CHECK-NOT: @__warn_memset_zero_len
+  __warn_memset_zero_len();
+}
+
 // Behavior of __builtin_os_log differs between platforms, so only test on X86
 #ifdef __x86_64__