[PDB/CodeView] Rename some classes.

In preparation for introducing writing capabilities for each of
these classes, I would like to adopt a Foo / FooRef naming
convention, where Foo indicates that the class can manipulate and
serialize Foos, and FooRef indicates that it is an immutable view of
an existing Foo.  In other words, Foo is a writer and FooRef is a
reader.  This patch names some existing readers to conform to the
FooRef convention, while offering no functional change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301810 91177308-0d34-0410-b5e6-96231b3b80d8

lib/DebugInfo/CodeView/ModuleDebugFragmentVisitor.cpp

@@ -24,21 +24,21 @@ Error llvm::codeview::visitModuleDebugFragment(
   BinaryStreamReader Reader(R.getRecordData());
   switch (R.kind()) {
   case ModuleDebugFragmentKind::Lines: {
-    ModuleDebugLineFragment Fragment;
+    ModuleDebugLineFragmentRef Fragment;
     if (auto EC = Fragment.initialize(Reader))
       return EC;
 
     return V.visitLines(Fragment);
   }
   case ModuleDebugFragmentKind::FileChecksums: {
-    ModuleDebugFileChecksumFragment Fragment;
+    ModuleDebugFileChecksumFragmentRef Fragment;
     if (auto EC = Fragment.initialize(Reader))
       return EC;
 
     return V.visitFileChecksums(Fragment);
   }
   default: {
-    ModuleDebugUnknownFragment Fragment(R.kind(), R.getRecordData());
+    ModuleDebugUnknownFragmentRef Fragment(R.kind(), R.getRecordData());
     return V.visitUnknown(Fragment);
   }
   }