[Serialization] Use the SourceLocation::UIntTy instead of the raw type

for the offset, NFC

Author: hokein

clang/include/clang/Serialization/ASTReader.h

@@ -525,7 +525,7 @@ class ASTReader
   ContinuousRangeMap<unsigned, ModuleFile*, 64> GlobalSLocEntryMap;
 
   using GlobalSLocOffsetMapType =
-      ContinuousRangeMap<unsigned, ModuleFile *, 64>;
+      ContinuousRangeMap<SourceLocation::UIntTy, ModuleFile *, 64>;
 
   /// A map of reversed (SourceManager::MaxLoadedOffset - SLocOffset)
   /// SourceLocation offsets to the modules containing them.