[lldb] Fix a crash in PlatformAppleSimulator::GetCoreSimulatorPath when Xcode...

[lldb] Fix a crash in PlatformAppleSimulator::GetCoreSimulatorPath when Xcode developer directory can't be found

Summary:

`PlatformAppleSimulator::GetCoreSimulatorPath` currently checks if
`m_core_simulator_framework_path` wasn't set yet and then tries to calculate its
actual value. However, if `GetXcodeDeveloperDirectory` returns an invalid
FileSpec, `m_core_simulator_framework_path` is never assigned a value which
causes that the `return m_core_simulator_framework_path.getValue();` at the end
of the function will trigger an assert.

This patch just assigns an invalid FileSpec to `m_core_simulator_framework_path`
which seems what the calling code in `PlatformAppleSimulator::LoadCoreSimulator`
expects as an error value.

I assume this can be reproduces on machines that don't have an Xcode
installation, but this patch is mostly based on this backtrace I received from
someone else that tried to run the test suite:

```
Assertion failed: (hasVal), function getValue, file llvm/include/llvm/ADT/Optional.h, line 73.
[...]
3   libsystem_c.dylib             	0x00007fff682a1ac6 __assert_rtn + 314
4   liblldb.11.0.0git.dylib       	0x000000010b835931 PlatformAppleSimulator::GetCoreSimulatorPath() (.cold.1) + 33
5   liblldb.11.0.0git.dylib       	0x0000000107e92f11 PlatformAppleSimulator::GetCoreSimulatorPath() + 369
6   liblldb.11.0.0git.dylib       	0x0000000107e9383e void std::__1::__call_once_proxy<std::__1::tuple<PlatformAppleSimulator::LoadCoreSimulator()::$_1&&> >(void*) + 30
7   libc++.1.dylib                	0x00007fff654d5bea std::__1::__call_once(unsigned long volatile&, void*, void (*)(void*)) + 139
8   liblldb.11.0.0git.dylib       	0x0000000107e92019 PlatformAppleSimulator::LaunchProcess(lldb_private::ProcessLaunchInfo&) + 89
9   liblldb.11.0.0git.dylib       	0x0000000107e92be5 PlatformAppleSimulator::DebugProcess(lldb_private::ProcessLaunchInfo&, lldb_private::Debugger&, lldb_private::Target*, lldb_private::Status&) + 101
10  liblldb.11.0.0git.dylib       	0x0000000107cb044d lldb_private::Target::Launch(lldb_private::ProcessLaunchInfo&, lldb_private::Stream*) + 669
11  liblldb.11.0.0git.dylib       	0x000000010792c9c5 lldb::SBTarget::Launch(lldb::SBLaunchInfo&, lldb::SBError&) + 1109
12  liblldb.11.0.0git.dylib       	0x0000000107a92acd _wrap_SBTarget_Launch(_object*, _object*) + 477
13  org.python.python             	0x000000010681076f PyCFunction_Call + 321
14  org.python.python             	0x000000010689ee12 _PyEval_EvalFrameDefault + 7738
```

Reviewers: JDevlieghere, jasonmolenda

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D80997