[clang][AST] Make `getLocalOrImportedSubmoduleID` work with const `Module*`. NFC.

[dexonsmith]

LGTM.

[vsapsai]

Thanks for the review!

clang/include/clang/Serialization/ASTWriter.h

private:
   unsigned NumLexicalDeclContexts = 0;
 
   /// The number of visible declcontexts written to the AST
   /// file.
   unsigned NumVisibleDeclContexts = 0;
 
   /// A mapping from each known submodule to its ID number, which will
   /// be a positive integer.
-  llvm::DenseMap<Module *, unsigned> SubmoduleIDs;
+  llvm::DenseMap<const Module *, unsigned> SubmoduleIDs;
 
   /// A list of the module file extension writers.
   std::vector<std::unique_ptr<ModuleFileExtensionWriter>>
       ModuleFileExtensionWriters;
 
   /// Retrieve or create a submodule ID for this module.
   unsigned getSubmoduleID(Module *Mod);
 
void EmitRecordWithPath(unsigned Abbrev, RecordDataRef Record,
                           StringRef Path);
 
   /// Add a version tuple to the given record
   void AddVersionTuple(const VersionTuple &Version, RecordDataImpl &Record);
 
   /// Retrieve or create a submodule ID for this module, or return 0 if
   /// the submodule is neither local (a submodle of the currently-written module)
   /// nor from an imported module.
-  unsigned getLocalOrImportedSubmoduleID(Module *Mod);
+  unsigned getLocalOrImportedSubmoduleID(const Module *Mod);
 
   /// Note that the identifier II occurs at the given offset
   /// within the identifier table.
   void SetIdentifierOffset(const IdentifierInfo *II, uint32_t Offset);
 
   /// Note that the selector Sel occurs at the given offset
   /// within the method pool/selector table.
   void SetSelectorOffset(Selector Sel, uint32_t Offset);

clang/lib/Serialization/ASTWriter.cpp

if (SkippedRanges.size() > 0) {
 
     Record.clear();
     Record.push_back(PPD_SKIPPED_RANGES);
     Stream.EmitRecordWithBlob(PPESkippedRangeAbbrev, Record,
                               bytes(SerializedSkippedRanges));
   }
 }
 
-unsigned ASTWriter::getLocalOrImportedSubmoduleID(Module *Mod) {
+unsigned ASTWriter::getLocalOrImportedSubmoduleID(const Module *Mod) {
   if (!Mod)
     return 0;
 
-  llvm::DenseMap<Module *, unsigned>::iterator Known = SubmoduleIDs.find(Mod);
+  auto Known = SubmoduleIDs.find(Mod);
   if (Known != SubmoduleIDs.end())
     return Known->second;
 
   auto *Top = Mod->getTopLevelModule();
   if (Top != WritingModule &&
       (getLangOpts().CompilingPCH ||
        !Top->fullModuleNameIs(StringRef(getLangOpts().CurrentModule))))
     return 0;