Use actual Identifiers for private discriminators, rather than strings.

This is useful both for caching purposes and for comparison of discriminators
(something the debugger will need to do when looking up a particular decl).

No observable functionality change.

Swift SVN r21610

lib/Serialization/SerializedModuleLoader.cpp

@@ -415,10 +415,9 @@ StringRef SerializedASTFile::getFilename() const {
   return File.getModuleFilename();
 }
 
-void SerializedASTFile::getDiscriminatorForPrivateValue(
-    SmallVectorImpl<char> &buffer,
-    const ValueDecl *D) const {
-  StringRef discriminator = File.getDiscriminatorForPrivateValue(D);
+Identifier
+SerializedASTFile::getDiscriminatorForPrivateValue(const ValueDecl *D) const {
+  Identifier discriminator = File.getDiscriminatorForPrivateValue(D);
   assert(!discriminator.empty() && "no discriminator found for value");
-  buffer.append(discriminator.begin(), discriminator.end());
+  return discriminator;
 }