Rework the serialization schema for conformances and requirements.

We now schedule conformance emissions in basically the same way
we do for types and declarations, which means that we'll emit them
uniquely in the module file instead of redundantly at every use.
This should produce substantially smaller module files overall,
especially for modules that heavily use generics.  It also means
that we can remove all the unfortunate code to support using
different abbrev codes for them in different bitcode blocks.

Requirement lists are now emitted inline in the records that need
them instead of as trailing records.  I think this will improve
space usage, but mostly it assists in eliminating the problem
where abbrev codes are shared between blocks.

lib/Serialization/ModuleFile.h

@@ -55,8 +55,10 @@ class ModuleFile
   friend class DeclDeserializer;
   friend class TypeDeserializer;
   friend class SILDeserializer;
+  friend class ProtocolConformanceDeserializer;
   using Status = serialization::Status;
   using TypeID = serialization::TypeID;
+  using ProtocolConformanceID = serialization::ProtocolConformanceID;
 
   /// The core data of a serialized module file. This is accessed as immutable
   /// and thread-safe.
@@ -239,8 +241,8 @@ private:
   /// Local DeclContexts referenced by this module.
   MutableArrayRef<Serialized<DeclContext*>> LocalDeclContexts;
 
-  /// Normal protocol conformances referenced by this module.
-  MutableArrayRef<Serialized<NormalProtocolConformance *>> NormalConformances;
+  /// Protocol conformances referenced by this module.
+  MutableArrayRef<Serialized<ProtocolConformance *>> Conformances;
 
   /// SILLayouts referenced by this module.
   MutableArrayRef<Serialized<SILLayout *>> SILLayouts;
@@ -257,6 +259,11 @@ private:
   /// Substitution maps referenced by this module.
   MutableArrayRef<Serialized<SubstitutionMap>> SubstitutionMaps;
 
+  uint64_t
+  createLazyConformanceLoaderToken(ArrayRef<uint64_t> ids);
+  ArrayRef<ProtocolConformanceID>
+  claimLazyConformanceLoaderToken(uint64_t token);
+
   /// Represents an identifier that may or may not have been deserialized yet.
   ///
   /// If \c Ident is empty, the identifier has not been loaded yet.
@@ -389,14 +396,16 @@ private:
   GenericParamList *maybeReadGenericParams(DeclContext *DC);
 
   /// Reads a set of requirements from \c DeclTypeCursor.
-  void readGenericRequirements(SmallVectorImpl<Requirement> &requirements,
-                               llvm::BitstreamCursor &Cursor);
+  void deserializeGenericRequirements(ArrayRef<uint64_t> scratch,
+                                      unsigned &nextIndex,
+                                 SmallVectorImpl<Requirement> &requirements);
 
   /// Reads a set of requirements from \c DeclTypeCursor, returns the first
   /// error, if any.
   llvm::Error
-  readGenericRequirementsChecked(SmallVectorImpl<Requirement> &requirements,
-                                 llvm::BitstreamCursor &Cursor);
+  deserializeGenericRequirementsChecked(ArrayRef<uint64_t> scratch,
+                                        unsigned &nextIndex,
+                                   SmallVectorImpl<Requirement> &requirements);
 
   /// Read the requirement signature of a protocol, which consists of a list of
   /// generic requirements and a list of protocol typealias records.
@@ -847,25 +856,19 @@ public:
   llvm::Expected<SubstitutionMap>
   getSubstitutionMapChecked(serialization::SubstitutionMapID id);
 
-  /// Recursively reads a protocol conformance from the given cursor.
-  ProtocolConformanceRef readConformance(llvm::BitstreamCursor &Cursor,
-                                         GenericEnvironment *genericEnv =
-                                           nullptr);
+  /// Returns the protocol conformance for the given ID.
+  ProtocolConformanceRef
+  getConformance(serialization::ProtocolConformanceID id,
+                 GenericEnvironment *genericEnv = nullptr);
 
-  /// Recursively reads a protocol conformance from the given cursor,
-  /// returns the conformance or the first error.
+  /// Returns the protocol conformance for the given ID.
   llvm::Expected<ProtocolConformanceRef>
-  readConformanceChecked(llvm::BitstreamCursor &Cursor,
-                         GenericEnvironment *genericEnv = nullptr);
+  getConformanceChecked(serialization::ProtocolConformanceID id,
+                        GenericEnvironment *genericEnv = nullptr);
 
   /// Read a SILLayout from the given cursor.
   SILLayout *readSILLayout(llvm::BitstreamCursor &Cursor);
 
-  /// Read the given normal conformance from the current module file,
-  /// returns the conformance or the first error.
-  llvm::Expected<NormalProtocolConformance *>
-  readNormalConformanceChecked(serialization::NormalConformanceID id);
-
   /// Reads a foreign error convention from \c DeclTypeCursor, if present.
   Optional<ForeignErrorConvention> maybeReadForeignErrorConvention();