[Serialization] Remove unused declaration SLocCursorForID

The corresponding function body was removed by:

  commit 925296b
  Author: Douglas Gregor <dgregor@apple.com>
  Date:   Tue Jul 19 16:10:42 2011 +0000

clang/include/clang/Serialization/ASTReader.h

@@ -1390,7 +1390,6 @@ class ASTReader
   void ReadModuleOffsetMap(ModuleFile &F) const;
   void ParseLineTable(ModuleFile &F, const RecordData &Record);
   llvm::Error ReadSourceManagerBlock(ModuleFile &F);
-  llvm::BitstreamCursor &SLocCursorForID(int ID);
   SourceLocation getImportLocation(ModuleFile *F);
   void readIncludedFiles(ModuleFile &F, StringRef Blob, Preprocessor &PP);
   ASTReadResult ReadModuleMapFileBlock(RecordData &Record, ModuleFile &F,