[clangd] Enable async preambles by default

[kadircet]

Summary

We've been testing this internally for a couple weeks now and it seems
to be stable enough. Let's flip the flag before branch cut to increase testing
coverage and have enough time to revert if need be.

clang-tools-extra/clangd/ClangdServer.h

struct Options {
 
     /// AST caching policy. The default is to keep up to 3 ASTs in memory.
     ASTRetentionPolicy RetentionPolicy;
 
     /// Cached preambles are potentially large. If false, store them on disk.
     bool StorePreamblesInMemory = true;
     /// Reuse even stale preambles, and rebuild them in the background.
     /// This improves latency at the cost of accuracy.
-    bool AsyncPreambleBuilds = false;
+    bool AsyncPreambleBuilds = true;
 
     /// If true, ClangdServer builds a dynamic in-memory index for symbols in
     /// opened files and uses the index to augment code completion results.
     bool BuildDynamicSymbolIndex = false;
     /// Use a heavier and faster in-memory index implementation.
     bool HeavyweightDynamicSymbolIndex = true;
     /// If true, ClangdServer automatically indexes files in the current project
     /// on background threads. The index is stored in the project root.

clang-tools-extra/clangd/TUScheduler.h

struct Options {
     /// This tries to ensure we rebuild once the user stops typing.
     DebouncePolicy UpdateDebounce;
 
     /// Determines when to keep idle ASTs in memory for future use.
     ASTRetentionPolicy RetentionPolicy;
 
     /// Whether to run PreamblePeer asynchronously.
     /// No-op if AsyncThreadsCount is 0.
-    bool AsyncPreambleBuilds = false;
+    bool AsyncPreambleBuilds = true;
 
     /// Used to create a context that wraps each single operation.
     /// Typically to inject per-file configuration.
     /// If the path is empty, context sholud be "generic".
     std::function<Context(PathRef)> ContextProvider;
   };
 
   TUScheduler(const GlobalCompilationDatabase &CDB, const Options &Opts,