[modules] Further simplification and speedup of redeclaration chain loading.

Instead of eagerly deserializing a list of DeclIDs when we load a module file
and doing a binary search to find the redeclarations of a decl, store a list of
redeclarations of each chain before the first declaration and load it directly.

llvm-svn: 245789

clang/lib/Serialization/Module.cpp

@@ -40,7 +40,6 @@ ModuleFile::ModuleFile(ModuleKind Kind, unsigned Generation)
     LocalNumCXXBaseSpecifiers(0), CXXBaseSpecifiersOffsets(nullptr),
     LocalNumCXXCtorInitializers(0), CXXCtorInitializersOffsets(nullptr),
     FileSortedDecls(nullptr), NumFileSortedDecls(0),
-    RedeclarationsMap(nullptr), LocalNumRedeclarationsInMap(0),
     ObjCCategoriesMap(nullptr), LocalNumObjCCategoriesInMap(0),
     LocalNumTypes(0), TypeOffsets(nullptr), BaseTypeIndex(0)
 {}