[clangd] Switch to incomplete translation units

[krasimir]

Summary

This speeds up code completion. All the cool kids (ycmd) are doing it.

[bkramer]

lg

clang-tools-extra/trunk/clangd/ClangdUnit.cpp

ASTUnit::RemappedFile RemappedSource(
       FileName,
       llvm::MemoryBuffer::getMemBufferCopy(Contents, FileName).release());
 
   auto ArgP = &*ArgStrs.begin();
   Unit = std::unique_ptr<ASTUnit>(ASTUnit::LoadFromCommandLine(
       ArgP, ArgP + ArgStrs.size(), PCHs, Diags, ResourceDir,
       /*OnlyLocalDecls=*/false, /*CaptureDiagnostics=*/true, RemappedSource,
       /*RemappedFilesKeepOriginalName=*/true,
-      /*PrecompilePreambleAfterNParses=*/1, /*TUKind=*/TU_Complete,
+      /*PrecompilePreambleAfterNParses=*/1, /*TUKind=*/TU_Prefix,
       /*CacheCodeCompletionResults=*/true,
       /*IncludeBriefCommentsInCodeCompletion=*/true,
       /*AllowPCHWithCompilerErrors=*/true));
 }
 
 void ClangdUnit::reparse(StringRef Contents) {
   // Do a reparse if this wasn't the first parse.
   // FIXME: This might have the wrong working directory if it changed in the