[clangd] Bump index version number.

[hokein]

Summary

https://reviews.llvm.org/D89670 changed the Ref structure, we need to
bump the version to invalidate all stored stale data, otherwise we will
get Error while reading shard: malformed or truncated refs when
building the background index.

[kadircet]

Ah thanks, and sorry for missing it during the review :/

[nridge]

My apologies for not thinking of this.

clang-tools-extra/clangd/index/Serialization.cpp

 //   - srcs: information related to include graph
 //   - stri: string table
 //   - symb: symbols
 //   - refs: references to symbols
 
 // The current versioning scheme is simple - non-current versions are rejected.
 // If you make a breaking change, bump this version number to invalidate stored
 // data. Later we may want to support some backward compatibility.
-constexpr static uint32_t Version = 13;
+constexpr static uint32_t Version = 14;
 
 llvm::Expected<IndexFileIn> readRIFF(llvm::StringRef Data) {
   auto RIFF = riff::readFile(Data);
   if (!RIFF)
     return RIFF.takeError();
   if (RIFF->Type != riff::fourCC("CdIx"))
     return error("wrong RIFF filetype: {0}", riff::fourCCStr(RIFF->Type));
   llvm::StringMap<llvm::StringRef> Chunks;