Recommit r308327 3rd time: Add a warning for missing
'#pragma pack (pop)' and suspicious uses of '#pragma pack' in included files

The second recommit (r309106) was reverted because the "non-default #pragma
pack value chages the alignment of struct or union members in the included file"
warning proved to be too aggressive for external projects like Chromium
(https://bugs.chromium.org/p/chromium/issues/detail?id=749197). This recommit
makes the problematic warning a non-default one, and gives it the
-Wpragma-pack-suspicious-include warning option.

The first recommit (r308441) caused a "non-default #pragma pack value might
change the alignment of struct or union members in the included file" warning
in LLVM itself. This recommit tweaks the added warning to avoid warnings for
#includes that don't have any records that are affected by the non-default
alignment. This tweak avoids the previously emitted warning in LLVM.

Original message:

This commit adds a new -Wpragma-pack warning. It warns in the following cases:

- When a translation unit is missing terminating #pragma pack (pop) directives.
- When entering an included file if the current alignment value as determined
  by '#pragma pack' directives is different from the default alignment value.
- When leaving an included file that changed the state of the current alignment
  value.

rdar://10184173

Differential Revision: https://reviews.llvm.org/D35484

llvm-svn: 309386
diff --git a/clang/lib/Parse/ParsePragma.cpp b/clang/lib/Parse/ParsePragma.cpp
index 2627437..c5215ff 100644
--- a/clang/lib/Parse/ParsePragma.cpp
+++ b/clang/lib/Parse/ParsePragma.cpp
@@ -422,15 +422,20 @@
   assert(Tok.is(tok::annot_pragma_pack));
   PragmaPackInfo *Info =
     static_cast<PragmaPackInfo *>(Tok.getAnnotationValue());
-  SourceLocation PragmaLoc = ConsumeAnnotationToken();
+  SourceLocation PragmaLoc = Tok.getLocation();
   ExprResult Alignment;
   if (Info->Alignment.is(tok::numeric_constant)) {
     Alignment = Actions.ActOnNumericConstant(Info->Alignment);
-    if (Alignment.isInvalid())
+    if (Alignment.isInvalid()) {
+      ConsumeAnnotationToken();
       return;
+    }
   }
   Actions.ActOnPragmaPack(PragmaLoc, Info->Action, Info->SlotLabel,
                           Alignment.get());
+  // Consume the token after processing the pragma to enable pragma-specific
+  // #include warnings.
+  ConsumeAnnotationToken();
 }
 
 void Parser::HandlePragmaMSStruct() {