[clangd] Bump index version number.

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.

Differential Revision: https://reviews.llvm.org/D91131

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

@@ -418,7 +418,7 @@ readCompileCommand(Reader CmdReader, llvm::ArrayRef<llvm::StringRef> Strings) {
 // 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);