Resubmit r354706 with a fix for process launch.

When the debugger is run in sync mode, you need to
be able to tell whether a hijacked resume is for some
special purpose (like waiting for the SIGSTOP on attach)
or just to perform a synchronous resume.  Target::Launch was doing
that wrong, and that caused stop-hooks on process launch
in source files to behave incorrectly.

<rdar://problem/48115661>

Differential Revision: https://reviews.llvm.org/D58727

llvm-svn: 355213