[clangd] Add createIndex in dexp

[hokein]

Summary

This would allow easily injecting our internal customization.

Also updates the stale "symbol-collection-file" flag.

[sammccall]

clangd/index/dex/dexp/Dexp.cpp

256

createIndex seems a slightly odd name for this (as if writing new files or adding new data somehow): openIndex or so?

273

"to open the index"?

[hokein]

createIndex => openIndex

clangd/index/dex/dexp/Dexp.cpp

 using clang::clangd::loadIndex;
 using clang::clangd::Symbol;
 using clang::clangd::SymbolIndex;
 using namespace llvm;
 
 namespace {
 
 llvm::cl::opt<std::string>
-    SymbolCollection("symbol-collection-file",
-                     llvm::cl::desc("Path to the file with symbol collection"),
-                     llvm::cl::Positional, llvm::cl::Required);
+    IndexPath("index-path",
+              llvm::cl::desc("Path to the index"),
 
 static const std::string Overview = R"(
 This is an **experimental** interactive tool to process user-provided search
 queries over given symbol collection obtained via global-symbol-builder. The
 tool can be used to evaluate search quality of existing index implementations
 and manually construct non-trivial test cases.
 
 Type use "help" request to get information about the details.

 } CommandInfo[] = {
     {"find", "Search for symbols with fuzzyFind", llvm::make_unique<FuzzyFind>},
     {"lookup", "Dump symbol details by ID or qualified name",
      llvm::make_unique<Lookup>},
     {"refs", "Find references by ID or qualified name",
      llvm::make_unique<Refs>},
 };
 
+std::unique_ptr<SymbolIndex> createIndex(llvm::StringRef Index) {
+  return loadIndex(Index, /*URISchemes=*/{}, /*UseDex=*/true);
+}
+
 } // namespace
 
 int main(int argc, const char *argv[]) {
   llvm::cl::ParseCommandLineOptions(argc, argv, Overview);
   llvm::cl::ResetCommandLineParser(); // We reuse it for REPL commands.
   llvm::sys::PrintStackTraceOnErrorSignal(argv[0]);
 
   std::unique_ptr<SymbolIndex> Index;
   reportTime("Dex build", [&]() {
-    Index = loadIndex(SymbolCollection, /*URISchemes=*/{},
-                      /*UseDex=*/true);
+    Index = createIndex(IndexPath);
   });
 
   if (!Index) {
-    llvm::outs()
-        << "ERROR: Please provide a valid path to symbol collection file.\n";
+    llvm::outs() << "Failed to create an index.\n";
     return -1;
   }
 
   llvm::LineEditor LE("dexp");
 
   while (llvm::Optional<std::string> Request = LE.readLine()) {
     // Split on spaces and add required null-termination.
     std::replace(Request->begin(), Request->end(), ' ', '\0');