swift234

[test] Add an interpreter mode to the tests.

This takes all %target-run-simple-swift and %target-run-stdlib-swift
invocations and runs them using the interpreter instead. To enable this
mode, pass --param=interpret to lit.py; you can add this flag to the
LLVM_LIT_ARGS CMake setting (which defaults to "-sv").

This doesn't support separated %target-build / %target-run steps, nor
does it work with StdlibUnittest (which uses posix_spawn to run its
subtasks). But it's a start.

<rdar://problem/17938202>

Swift SVN r21391