[llvm-libtool-darwin] Parse and ignore -syslibroot

[keith]

Summary

cctools libtool allows you to link dynamic libraries by passing through
a number of arguments to ld64. Because of this the default arguments
libtool receives in Xcode contains arguments that only matter in that
case. This change ignores this argument, at least until we ever support
that dynamic use case, so that you can use llvm-libtool-darwin as a
drop-in replacement in Xcode for cctools libtool. There are more
arguments we could ignore for this case, but we can probably add those
as the use case comes up.

llvm/test/tools/llvm-libtool-darwin/ignored-options.test

+## This tests some flags are correctly parsed even when they're not handled
+
+# RUN: llvm-libtool-darwin -V -syslibroot foo | FileCheck %s
+# RUN: llvm-libtool-darwin -h | FileCheck --check-prefix=HELP %s
+
+# CHECK: Default target:
+
+# HELP-NOT: syslibroot

llvm/tools/llvm-libtool-darwin/llvm-libtool-darwin.cpp

static cl::opt<bool> NoWarningForNoSymbols(
     cl::desc("Do not warn about files that have no symbols"),
     cl::cat(LibtoolCategory), cl::init(false));
 
 static cl::opt<bool> WarningsAsErrors("warnings_as_errors",
                                       cl::desc("Treat warnings as errors"),
                                       cl::cat(LibtoolCategory),
                                       cl::init(false));
 
+static cl::opt<std::string> IgnoredSyslibRoot("syslibroot", cl::Hidden);
+
 static const std::array<std::string, 3> StandardSearchDirs{
     "/lib",
     "/usr/lib",
     "/usr/local/lib",
 };
 
 struct Config {
   bool Deterministic = true; // Updated by 'D' and 'U' modifiers.