Fix SDK selection using "platform select" when --sysroot/--version/--build options were specified

Summary:
This patch fixes SDK selection in the following case:
```
platform select remote-ios --sysroot "/Users/IliaK/Library/Developer/Xcode/iOS DeviceSupport/8.1.2 (12B440)" --build 12B440 --version 8.1.2
target create --arch arm64 "~/Project1.app"
```

Currently the lldb selects a first SDK version (in name order) in directory and then updates it after the device is connected. This approach ignores user's arguments and actually "platform select" command doesn't make sense.

After this patch, lldb takes a SDK which matches to user's arguments.

Reviewers: jasonmolenda, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits, clayborg, jasonmolenda, aemerson

Differential Revision: http://reviews.llvm.org/D8249

llvm-svn: 232017