Tests: Re-design skip-function-bodies.swift test.

The skip-function-bodies.swift test previously relied on `-emit-sorted-sil` to
control the order of SIL emission for matching CHECK: lines. I found that this
made the test too difficult to evolve, so I've re-written the test to instead
check SILGen's output by extracting just the `string_literal` lines and then
sorting them. This makes it much more straightforward to add new test cases at
the right location in the file.

There are a number of additional improvements to the test:
- SILGen without any function body skipping is now checked as a baseline to
  ensure that all the checks are in the right order. Previously, most of the
  negative CHECK lines were not properly ordered in the file, so they could have
  missed regressions.
- Every declaration in the test has a set of exhaustive CHECK lines covering
  each of the outputs explicitly. While more verbose, it should be clearer what
  is expected for every declaration.
- Module interfaces are emitted with the proper flags and are typechecked to
  verify they are valid.
- Superfluous diagnostics (e.g. unused variables) have been minimized to
  improve the experience of debugging failures.

Author: tshortli