Record original source range of generated source buffers into dumped files

This helps us establish where the code from a generated source buffer
would go, for external tools that don't understand serialized
diagnostics.

include/swift/Basic/SourceManager.h

@@ -129,6 +129,7 @@ public:
   SourceManager(llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS =
                     llvm::vfs::getRealFileSystem())
     : FileSystem(FS) {}
+  ~SourceManager();
 
   llvm::SourceMgr &getLLVMSourceMgr() {
     return LLVMSourceMgr;