[clangd] Enable completion index by default, limit results to 100.

[sammccall]

Summary

This should speed up global code completion by avoiding deserializing
preamble declarations to look up names. The tradeoff is memory usage.
Currently the index is fairly naive and may not be much faster, but there's lots
of performance headroom.

These two changes go together because results from the index get copied a couple
of times, so we should avoid it for huge sets.

Also the flag should be -completion-limit, rather than -limit-completion.

[ilya-biryukov]

LGTM

[ioeric]

Woohoo!

clang-tools-extra/trunk/clangd/tool/ClangdMain.cpp

static llvm::cl::opt<PCHStorageFlag> PCHStorage(
     llvm::cl::desc("Storing PCHs in memory increases memory usages, but may "
                    "improve performance"),
     llvm::cl::values(
         clEnumValN(PCHStorageFlag::Disk, "disk", "store PCHs on disk"),
         clEnumValN(PCHStorageFlag::Memory, "memory", "store PCHs in memory")),
     llvm::cl::init(PCHStorageFlag::Disk));
 
 static llvm::cl::opt<int> LimitCompletionResult(
-    "limit-completion",
+    "completion-limit",
     llvm::cl::desc("Limit the number of completion results returned by clangd. "
                    "0 means no limit."),
-    llvm::cl::init(0));
+    llvm::cl::init(100));
 
 static llvm::cl::opt<bool> RunSynchronously(
     "run-synchronously",
     llvm::cl::desc("Parse on main thread. If set, -j is ignored"),
     llvm::cl::init(false), llvm::cl::Hidden);
 
 static llvm::cl::opt<Path>
     ResourceDir("resource-dir",
static llvm::cl::opt<Path> TraceFile(
     "trace",
     llvm::cl::desc(
         "Trace internal events and timestamps in chrome://tracing JSON format"),
     llvm::cl::init(""), llvm::cl::Hidden);
 
 static llvm::cl::opt<bool> EnableIndexBasedCompletion(
     "enable-index-based-completion",
     llvm::cl::desc(
-        "Enable index-based global code completion (experimental). Clangd will "
-        "use index built from symbols in opened files"),
-    llvm::cl::init(false), llvm::cl::Hidden);
+        "Enable index-based global code completion. "
+        "Clang uses an index built from symbols in opened files"),
+    llvm::cl::init(true));
 
 static llvm::cl::opt<Path> YamlSymbolFile(
     "yaml-symbol-file",
     llvm::cl::desc(
         "YAML-format global symbol file to build the static index. Clangd will "
         "use the static index for global code completion.\n"
         "WARNING: This option is experimental only, and will be removed "
         "eventually. Don't rely on it."),