[serialization] Add support for generic functions.

This currently does a little dance to handle PolymorphicFunctionTypes.
These are currently implemented by referencing the GenericParamList of
an actual polymorphic function, so they can't be deserialized without
knowing which function they are attached to. To solve this,
PolymorphicFunctionTypes are serialized as regular FunctionTypes. Then,
when a generic function decl is being deserialized, it will rebuild
a PolymorphicFunctionType from the serialized type and its own generic
parameter list.

Requirements on the generic types are coming next.

Swift SVN r5983

lib/Serialization/ModuleFile.h

@@ -32,6 +32,7 @@ namespace llvm {
 namespace swift {
 class Decl;
 class DeclContext;
+class GenericParamList;
 class Module;
 class Pattern;
 class ValueDecl;
@@ -141,6 +142,12 @@ private:
   /// If the record at the cursor is not a pattern, returns null.
   Pattern *maybeReadPattern();
 
+  /// Reads a generic param list from \c DeclTypeCursor.
+  ///
+  /// If the record at the cursor is not a generic param list, returns null
+  /// without moving the cursor.
+  GenericParamList *maybeReadGenericParams(DeclContext *DC);
+
   /// Creates an array of types from the given IDs.
   ///
   /// The returned array is owned by the ASTContext.
@@ -159,18 +166,9 @@ private:
   /// Returns the decl context with the given ID, deserializing it if needed.
   DeclContext *getDeclContext(serialization::DeclID DID);
 
-  /// Controls how a Decl is deserialized.
-  enum DeclDeserializationOptions {
-    /// Don't set the decl's context. It will be set by the caller instead.
-    ///
-    /// This is useful for declarations that reference each other at
-    /// construction time.
-    SkipContext = 0x1
-  };
-
   /// Returns the decl with the given ID, deserializing it if needed.
   Decl *getDecl(serialization::DeclID DID,
-                DeclDeserializationOptions opts = {});
+                Optional<DeclContext *> ForcedContext = {});
 
   /// Returns the type with the given ID, deserializing it if needed.
   Type getType(serialization::TypeID TID);