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/AST/Module.cpp

@@ -1377,11 +1377,13 @@ ArtificialMainKind SourceFile::getArtificialMainKind() const {
   return ArtificialMainKind::None;
 }
 
-void
-SourceFile::getDiscriminatorForPrivateValue(SmallVectorImpl<char> &buffer,
-                                            const ValueDecl *D) const {
+Identifier
+SourceFile::getDiscriminatorForPrivateValue(const ValueDecl *D) const {
   assert(D->getDeclContext()->getModuleScopeContext() == this);
 
+  if (!PrivateDiscriminator.empty())
+    return PrivateDiscriminator;
+
   StringRef name = getFilename();
   if (name.empty()) {
     assert(1 == std::count_if(getParentModule()->getFiles().begin(),
@@ -1391,8 +1393,8 @@ SourceFile::getDiscriminatorForPrivateValue(SmallVectorImpl<char> &buffer,
     }) && "can't promise uniqueness if multiple source files are nameless");
 
     // We still need a discriminator.
-    buffer.push_back('_');
-    return;
+    PrivateDiscriminator = getASTContext().Id_Underscore;
+    return PrivateDiscriminator;
   }
 
   // Use a hash of the basename of the source file as our discriminator.
@@ -1406,7 +1408,7 @@ SourceFile::getDiscriminatorForPrivateValue(SmallVectorImpl<char> &buffer,
   hash.final(result);
 
   // Make sure the whole thing is a valid identifier.
-  buffer.push_back('_');
+  SmallString<33> buffer{"_"};
 
   // Write the hash as a hex string.
   // FIXME: This should go into llvm/ADT/StringExtras.h.
@@ -1416,7 +1418,9 @@ SourceFile::getDiscriminatorForPrivateValue(SmallVectorImpl<char> &buffer,
     buffer.push_back(llvm::hexdigit(byte >> 4, /*lowercase=*/false));
     buffer.push_back(llvm::hexdigit(byte & 0xF, /*lowercase=*/false));
   }
-  // FIXME: Cache the result?
+
+  PrivateDiscriminator = getASTContext().getIdentifier(buffer);
+  return PrivateDiscriminator;
 }
 
 //===----------------------------------------------------------------------===//