Revert "Summon the eldritch horror "EagerDeserializationDecls" from the bowels of history."

This reverts commit dc98e17d84b991b6be8b8feb5e0d05aad24f52a4.

I believe this commit was causing test failures on:
  IRGen/c_layout.sil
  IRGen/existentials.sil
It also recreates the file lib/Serialization/ModuleFormat.h,
which really can't have been intended.

Swift SVN r23732

lib/Serialization/ModuleFile.cpp

@@ -323,8 +323,6 @@ ModuleFile::readObjCMethodTable(ArrayRef<uint64_t> fields, StringRef blobData) {
                                              base + sizeof(uint32_t), base));
 }
 
-
-
 bool ModuleFile::readIndexBlock(llvm::BitstreamCursor &cursor) {
   cursor.EnterSubBlock(INDEX_BLOCK_ID);
 
@@ -380,11 +378,6 @@ bool ModuleFile::readIndexBlock(llvm::BitstreamCursor &cursor) {
         break;
       case index_block::OBJC_METHODS:
         ObjCMethods = readObjCMethodTable(scratch, blobData);
-        break;
-      case index_block::FORCE_DESERIALIZATION:
-        std::copy(scratch.begin(), scratch.end(),
-                  std::back_inserter(EagerDeserializationDecls));
-        
         break;
       default:
         // Unknown index kind, which this version of the compiler won't use.
@@ -894,13 +887,6 @@ bool ModuleFile::associateWithFileContext(FileUnit *file, SourceLoc diagLoc) {
   if (Bits.HasUnderlyingModule)
     (void)getModule(FileContext->getParentModule()->Name);
 
-  // Process decls we know we want to eagerly deserialize.
-  if (!EagerDeserializationDecls.empty()) {
-    for (DeclID DID : EagerDeserializationDecls)
-      getDecl(DID);
-    typeCheckExternalDefinitions(*FileContext);
-  }
-
   return getStatus() == ModuleStatus::Valid;
 }