prefer File.join over manual string formatting
Add support for linking C and Swift object files

- Compile C sources
- Copy headers into framework bundle
- Define a framework module map
- Link both C and Swift object files into framework executable
allow swift code to reference C code in the same framework

- with this change, Hello.swift can no longer be compiled as part of the
  'Hello' swiftmodule, and so stuff is broken there
- also, the definition of whether "swiftc_task" is needed may not be
  quite right, as it has no notion of the .swiftmodule or .swiftdoc
  files.