[clangd] Don't clone SymbolSlab::Builder arenas when finalizing.

[sammccall]

Summary

SymbolSlab::Builder has an arena to store strings of owned symbols, and
deduplicates them. build() copies all the strings and deduplicates them again!
This is potentially useful: we may have overwritten a symbol and
rendered some strings unreachable.

However in practice this is not the case. When testing on a variety of
files in LLVM (e.g. SemaExpr.cpp), the strings for the full preamble
index are 3MB and shrink by 0.4% (12KB). For comparison the serializde
preamble is >50MB.
There are also hundreds of smaller slabs (file sharding) that do not shrink at
all.

CPU time spent on this is significant (something like 3-5% of buildPreamble).
We're better off not bothering.

[sammccall]

My first take on this was to keep track in various ways of whether GC was required (statically known at callsite, or have builder track whether symbols were overwritten).
However these failed to identify a bunch of cases where the arena wasn't shrinking, and this seems much simpler.

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

 SymbolSlab SymbolSlab::Builder::build() && {
   // Sort symbols into vector so the slab can binary search over them.
   std::vector<Symbol> SortedSymbols;
   SortedSymbols.reserve(Symbols.size());
   for (auto &Entry : Symbols)
     SortedSymbols.push_back(std::move(Entry.second));
   llvm::sort(SortedSymbols,
              [](const Symbol &L, const Symbol &R) { return L.ID < R.ID; });
-  // We may have unused strings from overwritten symbols. Build a new arena.
-  llvm::BumpPtrAllocator NewArena;
-  llvm::UniqueStringSaver Strings(NewArena);
-  for (auto &S : SortedSymbols)
-    own(S, Strings);
-  return SymbolSlab(std::move(NewArena), std::move(SortedSymbols));
+  // We may have unused strings from overwritten symbols.
+  // In practice, these are extremely small, it's not worth compacting.
+  return SymbolSlab(std::move(Arena), std::move(SortedSymbols));
 }
 
 llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const SymbolSlab &Slab) {
   OS << "{";
   llvm::StringRef Sep = "";
   for (const auto &S : Slab) {
     OS << Sep << S;
     Sep = ", ";
   }
   OS << "}";
   return OS;
 }
 } // namespace clangd
 } // namespace clang