Clear any LLVMArgs when initializing the CompilerInvocation for SourceKit's ASTManager

SourceKit doesn't use them and if any unrecognised LLVM options are
passed to llvm::cl::ParseCommandLineOptions() it calls exit(), bringing
down SourceKit.

Also use fprintf instead of llvm::errs() in Logging.cpp as it uses a
global C++ object that had already been destructed when logging the
above failure.

Resolves rdar://problem/38314383

Author: Nathan Hawes

test/SourceKit/CursorInfo/ignore-llvm-args.swift

@@ -0,0 +1,6 @@
+_ = ""
+
+// rdar://problem/38314383
+// RUN: %sourcekitd-test -req=cursor -offset=0 %s -- -Xllvm -aarch64-use-tbi %s | %FileCheck %s
+
+// CHECK: <empty cursor info>

tools/SourceKit/lib/Support/Logging.cpp

@@ -63,7 +63,7 @@ Logger::~Logger() {
   OS << llvm::format("%7.4f] ", TR.getWallTime() - sBeginTR.getWallTime());
   OS << Msg.str();
 
-  llvm::errs() << LoggerName << ": " << LogMsg.str() << '\n';
+  fprintf(stderr, "%s: %s", LoggerName.c_str(), LogMsg.c_str());
 
 #if __APPLE__
   // Use the Apple System Log facility.

tools/SourceKit/lib/SwiftLang/SwiftASTManager.cpp

@@ -459,6 +459,9 @@ bool SwiftASTManager::initCompilerInvocation(CompilerInvocation &Invocation,
   // SwiftONoneSupport module.
   FrontendOpts.RequestedAction = FrontendOptions::ActionType::Typecheck;
 
+  // We don't care about LLVMArgs
+  FrontendOpts.LLVMArgs.clear();
+
   return false;
 }