Linking fails with the message "illegal thread-local reference to regular symbol" when attempting to create a dylib which references an external TLS variable declared with `__thread`.

In `foo.cc`:

    extern __thread int x;

    void foo() { ++x; }

The compile step is (this works fine):

    $CXX -stdlib=libc++ -std=c++11 -fvisibility=hidden -fPIC -o foo.cc.o -c foo.cc

The linking step (where the error is thrown):

    $CXX -stdlib=libc++ -std=c++11 -fvisibility=hidden -dynamiclib -Wl,-undefined,dynamic_lookup -o libfoo.dylib foo.cpp.o

Now this works fine in gcc (5 and 4.9.3) on the same platform. I understand there are differences in the implementation of `__thread`, but I cannot find any documentation addressing this specific issue anywhere, so I assume this is a bug.

Version info:

    $ $CXX --version
    Apple LLVM version 7.0.2 (clang-700.1.81)
    Target: x86_64-apple-darwin14.5.0
    Thread model: posix

    $ ld -v
    @(#)PROGRAM:ld  PROJECT:ld64-253.9
    configured to support archs: armv6 armv7 armv7s arm64 i386 x86_64 x86_64h armv6m armv7k armv7m armv7em (tvOS)
    LTO support using: Apple LLVM 7.0.2 (clang-700.1.81)