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

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;
 }