[Serialization] Add precomputed layout table for hidden types

Add the module-format machinery that lets a Swift library record the
physical layout of hidden types (currently limited to C types imported via internal bridging header).
into binary modules, so downstream consumers can pull the layouts of these hidden types without
loading the internal dependency.

To test this, this change also added a frontend action to print hidden types' layouts
from both the module under compilation and all the modules being imported.

lib/Serialization/ModuleFileSharedCore.cpp

@@ -1125,6 +1125,74 @@ bool ModuleFileSharedCore::readIndexBlock(llvm::BitstreamCursor &cursor) {
   return false;
 }
 
+bool ModuleFileSharedCore::readHiddenTypeLayoutsBlock(
+    llvm::BitstreamCursor &cursor) {
+  if (llvm::Error Err = cursor.EnterSubBlock(HIDDEN_TYPE_LAYOUTS_BLOCK_ID)) {
+    consumeError(std::move(Err));
+    return false;
+  }
+
+  SmallVector<uint64_t, 8> scratch;
+  StringRef blobData;
+
+  while (!cursor.AtEndOfStream()) {
+    Expected<llvm::BitstreamEntry> maybeEntry = cursor.advance();
+    if (!maybeEntry) {
+      consumeError(maybeEntry.takeError());
+      return false;
+    }
+    llvm::BitstreamEntry entry = maybeEntry.get();
+    switch (entry.Kind) {
+    case llvm::BitstreamEntry::EndBlock:
+      return true;
+
+    case llvm::BitstreamEntry::Error:
+      return false;
+
+    case llvm::BitstreamEntry::SubBlock:
+      // Unexpected sub-block; skip for forward compatibility.
+      if (cursor.SkipBlock())
+        return false;
+      break;
+
+    case llvm::BitstreamEntry::Record: {
+      scratch.clear();
+      blobData = {};
+      Expected<unsigned> maybeKind =
+          cursor.readRecord(entry.ID, scratch, &blobData);
+      if (!maybeKind) {
+        consumeError(maybeKind.takeError());
+        return false;
+      }
+      unsigned kind = maybeKind.get();
+      if (kind != hidden_type_layouts_block::HIDDEN_TYPE_LAYOUT) {
+        // Unknown record kind, ignore for forward compatibility.
+        break;
+      }
+      if (scratch.size() < 5)
+        return false;
+
+      AbstractTypeLayout layout;
+      layout.size = scratch[0];
+      layout.alignment = scratch[1];
+      layout.stride = scratch[2];
+      layout.bitwiseCopyable = scratch[3] != 0;
+      layout.isOpaque = scratch[4] != 0;
+      layout.mangledName = blobData.str();
+
+      // StringMap owns its keys (copies blobData). If a duplicate entry
+      // appears the module is malformed.
+      auto result = HiddenTypeLayouts.try_emplace(blobData, std::move(layout));
+      if (!result.second)
+        return false;
+      break;
+    }
+    }
+  }
+
+  return false;
+}
+
 std::unique_ptr<ModuleFileSharedCore::SerializedDeclCommentTable>
 ModuleFileSharedCore::readDeclCommentTable(ArrayRef<uint64_t> fields,
                                  StringRef blobData) const {
@@ -1901,6 +1969,14 @@ ModuleFileSharedCore::ModuleFileSharedCore(
       break;
     }
 
+    case HIDDEN_TYPE_LAYOUTS_BLOCK_ID: {
+      if (!readHiddenTypeLayoutsBlock(cursor)) {
+        info.status = error(Status::Malformed);
+        return;
+      }
+      break;
+    }
+
     case SIL_INDEX_BLOCK_ID: {
       // Save the cursor.
       SILIndexCursor = cursor;