[Serialization] Use llvm::map_range (NFC) (#135990)

Author: kazutakahirata

Diff for: clang/lib/Serialization/MultiOnDiskHashTable.h

@@ -103,11 +103,9 @@ template<typename Info> class MultiOnDiskHashTable {
 
   /// The current set of on-disk tables.
   table_range tables() {
-    auto Begin = Tables.begin(), End = Tables.end();
-    if (getMergedTable())
-      ++Begin;
-    return llvm::make_range(llvm::map_iterator(Begin, AsOnDiskTable()),
-                            llvm::map_iterator(End, AsOnDiskTable()));
+    unsigned DropBegin = getMergedTable() ? 1 : 0;
+    return llvm::map_range(llvm::drop_begin(Tables, DropBegin),
+                           AsOnDiskTable());
   }
 
   MergedTable *getMergedTable() const {