All bors builds failing on xcode 7.3

[scottmcm]

Happened for these PRs so far:
#46226 (comment)
#45969 (comment)
#46256 (comment)
#46280 (comment)
#46287 (comment)
#46290 (comment)
#46299 (comment)
#46322 (comment)
#46343 (comment)

[00:12:56] In file included from /Users/travis/build/rust-lang/rust/src/llvm/lib/Support/ThreadPool.cpp:14:
[00:12:56] In file included from /Users/travis/build/rust-lang/rust/src/llvm/include/llvm/Support/ThreadPool.h:30:
[00:12:56] /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/future:1447:23: error: 'future_error' is unavailable: introduced in macOS 10.8
[00:12:56]                       future_error(make_error_code(future_errc::broken_promise))
[00:12:56]                       ^
[00:12:56] /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/future:502:63: note: 'future_error' has been explicitly marked unavailable here
[00:12:56] class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_FUTURE_ERROR future_error
[00:12:56]                                                               ^
[00:12:56] /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/future:1621:23: error: 'future_error' is unavailable: introduced in macOS 10.8
[00:12:56]                       future_error(make_error_code(future_errc::broken_promise))
[00:12:56]                       ^
[00:12:56] /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/future:502:63: note: 'future_error' has been explicitly marked unavailable here
[00:12:56] class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_FUTURE_ERROR future_error
[00:12:56]                                                               ^
[00:12:56] 2 errors generated.

[kennytm]

Thanks for the report. This is a Travis CI issue, currently tracked in travis-ci/travis-ci#8821. A temporary workaround that disables all Xcode 7.3 (#46360) is currently testing.