[clangd] Bump index version number.

[hokein]

Summary

Though we don't have new changes to the index format, we have changes to
symbol collector, e.g. collect marcos, spelled references. Bump the
version to force background-index to rebuild.

[kadircet]

i've thought change introducing RefKind::Spelled would've already bumped it. Please make sure to bump it only if it didn't

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 = 12;
+constexpr static uint32_t Version = 13;
 
 llvm::Expected<IndexFileIn> readRIFF(llvm::StringRef Data) {
   auto RIFF = riff::readFile(Data);
   if (!RIFF)
     return RIFF.takeError();
   if (RIFF->Type != riff::fourCC("CdIx"))
     return makeError("wrong RIFF type");
   llvm::StringMap<llvm::StringRef> Chunks;