Allow clients to control the exact path that is used to launch processes by adding new calls to SBLaunchInfo.

The new calls are:

SBFileSpec
SBLaunchInfo::GetExecutableFile ();

void
SBLaunchInfo::SetExecutableFile (SBFileSpec exe_file, bool add_as_first_arg);

<rdar://problem/16833939>



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@208245 91177308-0d34-0410-b5e6-96231b3b80d8