[lldb] SBTarget::AddModule do all searches by UUID, set Target arch

Make SBTarget::AddModule possibly call out to an external program to
find the binary by UUID if it can't be found more easily, the same
way `target modules add -u ...` works from the commandline.

If the Target does not have an architecture set yet, use the
Module's Arch to initialize it.  Allows an API writer to create
a target with no arch, and inherit it from the first binary they
load with AddModules.

Differential Revision: https://reviews.llvm.org/D157659
rdar://113657555