Fix Target::Launch in case of synchronous execution

Summary: This patch fixes Target::Launch in case of synchronous execution.

Test Plan:
# Create file with source commands:
```
$ cat start_script
target create ~/p/hello
process launch -s
continue
```

# Run lldb and execute "command source -c 0 -s 0 start_script":
```
$ bin/lldb
(lldb) command source -c 0 -s 0 start_script
Executing commands in '/Users/IliaK/p/llvm/build_ninja/start_script'.
(lldb) target create ~/p/hello
Current executable set to '~/p/hello' (x86_64).
(lldb) process launch -s
Process 92028 stopped
* thread #1: tid = 0x26731, 0x00007fff5fc01000 dyld`_dyld_start, stop reason = signal SIGSTOP
    frame #0: 0x00007fff5fc01000 dyld`_dyld_start
dyld`_dyld_start:
->  0x7fff5fc01000 <+0>: popq   %rdi
    0x7fff5fc01001 <+1>: pushq  $0x0
    0x7fff5fc01003 <+3>: movq   %rsp, %rbp
    0x7fff5fc01006 <+6>: andq   $-0x10, %rsp
(lldb) Process 92028 launched: '/Users/IliaK/p/hello' (x86_64)
(lldb) continue
'
` - it's \ni=1
j=2
x=3
y=4
argc: /Users/IliaK/p/hello
argc: (null)
Process 92028 resuming
Process 92028 exited with status = 0 (0x00000000)

(lldb)
```

was:
```
$ bin/lldb
(lldb) command source -c 0 -s 0 start_script
Executing commands in '/Users/IliaK/p/llvm/build_ninja/start_script'.
(lldb) target create ~/p/hello
Current executable set to '~/p/hello' (x86_64).
(lldb) process launch -s
Process 92100 launched: '/Users/IliaK/p/hello' (x86_64)
(lldb) continue
error: Process must be launched.
Process 92100 stopped
* thread #1: tid = 0x2699a, 0x00007fff5fc01000 dyld`_dyld_start, stop reason = signal SIGSTOP
    frame #0: 0x00007fff5fc01000 dyld`_dyld_start
dyld`_dyld_start:
->  0x7fff5fc01000 <+0>: popq   %rdi
    0x7fff5fc01001 <+1>: pushq  $0x0
    0x7fff5fc01003 <+3>: movq   %rsp, %rbp
    0x7fff5fc01006 <+6>: andq   $-0x10, %rsp
(lldb)
```

Reviewers: jingham, clayborg

Reviewed By: clayborg

Subscribers: labath, lldb-commits, clayborg, jingham

Differential Revision: http://reviews.llvm.org/D8541

llvm-svn: 233022