compiler ICE when unable to find iOS SDK

[cardoe]

The compiler has an internal compiler error when it is unable to find the iOS SDK. This should probably be caught with a little friendlier error message.

$ rustc --emit llvm-ir --target x86_64-apple-ios -o x86_64-apple-ios.ll test.rs
error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rustlang/rust/blob/master/CONTRIBUTING.md#bug-reports
note: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'failed to get iphonesimulator SDK path: No such file or directory (os error 2)', ../src/librustc_back/target/apple_ios_base.rs:59

[cardoe]

This is a duplicate of #29812

[cardoe]

This should likely improve with #34980.