[clang] NFC: Remove GCC 5.1 workaround in FileEntry

[jansvoboda11]

Summary

We no longer support GCC 5.1 (D122976), so let's remove the workaround in FileEntry.

clang/include/clang/Basic/FileEntry.h

public:
 
   /// Type stored in the StringMap.
   struct MapValue {
     /// The pointer at another MapEntry is used when the FileManager should
     /// silently forward from one name to another, which occurs in Redirecting
     /// VFSs that use external names. In that case, the \c FileEntryRef
     /// returned by the \c FileManager will have the external name, and not the
     /// name that was used to lookup the file.
-    ///
-    /// The second type is really a `const MapEntry *`, but that confuses
-    /// gcc5.3.  Once that's no longer supported, change this back.
-    llvm::PointerUnion<FileEntry *, const void *> V;
+    llvm::PointerUnion<FileEntry *, const MapEntry *> V;
 
     /// Directory the file was found in. Set if and only if V is a FileEntry.
     OptionalDirectoryEntryRef Dir;
 
     MapValue() = delete;
     MapValue(FileEntry &FE, DirectoryEntryRef Dir) : V(&FE), Dir(Dir) {}
     MapValue(MapEntry &ME) : V(&ME) {}
   };
public:
   }
 
   /// Expose the underlying MapEntry to simplify packing in a PointerIntPair or
   /// PointerUnion and allow construction in Optional.
   const clang::FileEntryRef::MapEntry &getMapEntry() const { return *ME; }
 
   /// Retrieve the base MapEntry after redirects.
   const MapEntry &getBaseMapEntry() const {
-    const MapEntry *ME = this->ME;
-    while (const void *Next = ME->second->V.dyn_cast<const void *>())
-      ME = static_cast<const MapEntry *>(Next);
-    return *ME;
+    const MapEntry *Base = ME;
+    while (const auto *Next = Base->second->V.dyn_cast<const MapEntry *>())
+      Base = Next;
+    return *Base;
   }
 
 private:
   friend class FileMgr::MapEntryOptionalStorage<FileEntryRef>;
   struct optional_none_tag {};
 
   // Private constructor for use by OptionalStorage.
   FileEntryRef(optional_none_tag) : ME(nullptr) {}

clang/lib/Basic/FileManager.cpp

FileEntryRef FileManager::getVirtualFileRef(StringRef Filename, off_t Size,
 
   // See if there is already an entry in the map for an existing file.
   auto &NamedFileEnt = *SeenFileEntries.insert(
       {Filename, std::errc::no_such_file_or_directory}).first;
   if (NamedFileEnt.second) {
     FileEntryRef::MapValue Value = *NamedFileEnt.second;
     if (LLVM_LIKELY(Value.V.is<FileEntry *>()))
       return FileEntryRef(NamedFileEnt);
-    return FileEntryRef(*reinterpret_cast<const FileEntryRef::MapEntry *>(
-        Value.V.get<const void *>()));
+    return FileEntryRef(*Value.V.get<const FileEntryRef::MapEntry *>());
   }
 
   // We've not seen this before, or the file is cached as non-existent.
   ++NumFileCacheMisses;
   addAncestorsAsVirtualDirs(Filename);
   FileEntry *UFE = nullptr;
 
   // Now that all ancestors of Filename are in the cache, the