Add a new SBThread::SafeToCallFunctions API; this calls over to
the SystemRuntime to check if a thread will have any problems 
performing an inferior function call so the driver can skip
making that function call on that thread.  Often the function
call can be executed on another thread instead.
<rdar://problem/16777874> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@208732 91177308-0d34-0410-b5e6-96231b3b80d8