[PATCH] Insert poisoned paddings between fields in C++ classes.

[Richard Smith]

LGTM

================
Comment at: include/clang/Basic/DiagnosticFrontendKinds.td:50-56
@@ +49,9 @@
+def remark_sanitize_address_insert_extra_padding_accepted : Remark<
+    "-fsanitize-address-field-padding applied to %0">, BackendInfo,
+    InGroup<SanitizeAddressRemarks>;
+def remark_sanitize_address_insert_extra_padding_rejected : Remark<
+    "-fsanitize-address-field-padding ignored for %0 because it "
+    "%select{is not C++|is packed|is a union|is trivially copyable|"
+    "has trivial destructor|is standard layout|is in a blacklisted file|"
+    "is blacklisted}1">, BackendInfo,
+    InGroup<SanitizeAddressRemarks>;
----------------
I don't think these should be `BackendInfo`, because they're reflecting frontend semantics changes. They should probably be just `ShowInSystemHeader`.

================
Comment at: lib/AST/Decl.cpp:3629-3630
@@ +3628,4 @@
+  int ReasonToReject = -1;
+  if (!CXXRD)
+    ReasonToReject = 0;  // is not C++.
+  else if (CXXRD->hasAttr<PackedAttr>())
----------------
It would seem prudent to also test whether the type is in an `extern "C"` linkage specification in C++ here:

    if (!CXXRD || CXXRD->isExternCContext())

http://reviews.llvm.org/D5687