Invert ArchSpec<->Platform dependency

Summary:
ArchSpec::SetTriple was taking a Platform as an argument, and used it to
fill in missing pieces of the specified triple. I invert the dependency
by moving this code to other classes. For this purpose, I've created
three new functions.
- HostInfo::GetAugmentedArchSpec: fills in the triple using the host
  platform (this used to be implemented by passing a null platform
  pointer). By putting this code in the Host module, we can provide a
  way to anyone who does not have a platform instance (lldb-server) an
  easy way to get Host data.
- Platform::GetAugmentedArchSpec: if you have a platform instance, you
  can call this to let it fill in the triple.
- static Platform::GetAugmentedArchSpec: implements the "if platform ==
  0 then use_host() else use_platform()" part.

Reviewers: zturner, jingham, clayborg

Subscribers: mgorny, javed.absar, lldb-commits

Differential Revision: https://reviews.llvm.org/D39387

llvm-svn: 316987