[clangd] Support -fallback-style, similar to clang-format.

[kadircet]

LGTM

Putting http://lists.llvm.org/pipermail/clangd-dev/2019-May/000411.html here for context.

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

 #include "Features.inc"
 #include "Path.h"
 #include "Protocol.h"
 #include "Trace.h"
 #include "Transport.h"
 #include "index/Background.h"
 #include "index/Serialization.h"
 #include "clang/Basic/Version.h"
+#include "clang/Format/Format.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Program.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cstdlib>
llvm::cl::desc(
         ".clang-tidy files). Only meaningful when -clang-tidy flag is on."),
     llvm::cl::init(""));
 
 static llvm::cl::opt<bool> EnableClangTidy(
     "clang-tidy",
     llvm::cl::desc("Enable clang-tidy diagnostics."),
     llvm::cl::init(true));
 
+static llvm::cl::opt<std::string>
+    FallbackStyle("fallback-style",
+                  llvm::cl::desc("clang-format style to apply by default when "
+                                 "no .clang-format file is found"),
+                  llvm::cl::init(clang::format::DefaultFallbackStyle));
+
 static llvm::cl::opt<bool> SuggestMissingIncludes(
     "suggest-missing-includes",
     llvm::cl::desc("Attempts to fix diagnostic errors caused by missing "
                    "includes using index."),
     llvm::cl::init(true));
 
 static llvm::cl::opt<OffsetEncoding> ForceOffsetEncoding(
     "offset-encoding",
if (!RunSynchronously && WorkerThreadsCount == 0) {
     return 1;
   }
 
   if (RunSynchronously) {
     if (WorkerThreadsCount.getNumOccurrences())
       llvm::errs() << "Ignoring -j because -run-synchronously is set.\n";
     WorkerThreadsCount = 0;
   }
+  if (FallbackStyle.getNumOccurrences())
+    clang::format::DefaultFallbackStyle = FallbackStyle.c_str();
 
   // Validate command line arguments.
   llvm::Optional<llvm::raw_fd_ostream> InputMirrorStream;
   if (!InputMirrorFile.empty()) {
     std::error_code EC;
     InputMirrorStream.emplace(InputMirrorFile, /*ref*/ EC,
                               llvm::sys::fs::FA_Read | llvm::sys::fs::FA_Write);
     if (EC) {