On Mac OS X the Objective-C runtime (libobjc) has many critical

dispatch functions that are implemented in hand-written assembly.
There is also hand-written eh_frame instructions for unwinding
from these functions.

Normally we don't use eh_frame instructions for the currently
executing function, prefering the assembly instruction profiling
method.  But in these hand-written dispatch functions, the
profiling is doomed and we should use the eh_frame instructions.

Unfortunately there's no easy way to flag/extend the eh_frame/debug_frame
sections to annotate if the unwind instructions are accurate at
all addresses ("asynchronous") or if they are only accurate at locations
that can throw an exception ("synchronous" and the normal case for 
gcc/clang generated eh_frame/debug_frame CFI).

<rdar://problem/10508134>

llvm-svn: 146551