Revert "Use _Unwind_Backtrace on Apple platforms."

This reverts commit 63165f6ae3bac1623be36d4b3ce63afa1d51a30a.

After making this change, I discovered that _Unwind_Backtrace is
unable to unwind past a signal handler after an assertion failure.
I filed a bug report about that issue in rdar://29866587 but even if
we get a fix soon, it will be awhile before it get released.

llvm-svn: 291207