swift234

Start the swiftReflection library

- Nearly done: TypeRefs and the mangled name decoder.

- Add the swift-reflection-test tool.
  The field reflection pipeline is roughly:
  - Decode type references
  - Substitute generic parameters
  - Calculate sizes and offsets

  There is currently only one action in the tool, which will test the
  *Decode* part of the pipeline: `dump-reflection-section`. This reads
  the *swift3_reflect section from an object file and dumps the decoded
  type references for all of the stored properties and enum cases in the
  file.
  - TODO: Write tests with various type arrangements to exercise the
    decoder - there are likely some holes in the decoder still since the
    AST mangler is quite rich in its kinds.

  TODO: The next test mode, `dump-field-types`, will do the following:
  1. Launch a swift executable with a canned stopping point
  2. Get the address of a heap object instance of interest
  3. Dump the fully substituted typerefs of all of the stored properties
  or enum case payloads.

  That test mode will be more involved since it will attach to another
  process and need to read from its address space but will test the
  entire out-of-process reflection pipeline in a controlled environment.
  We can maybe take this test a step further, with an option or a new
  test mode, that prints the entire heap reference graph rooted at that
  object of interest, in order to test the ability to detect reference
  cycles, for example.