swift234

[driver] Add -lldb-repl and -integrated-repl modes.

Since LLDB is taking over as the REPL for Swift, we eventually want "swift"
and "swift -repl" to invoke "lldb --repl" rather than the frontend. However,
we only want to do this if the LLDB that's present is related to the Swift
that's present -- we don't want to invoke some random LLDB on the system
or in some other Xcode installation. Therefore, Swift searches for LLDB--
first next to the driver, then in the usr/bin/ outside of a toolchain--
before choosing to use it.

If the user just passes -repl and an LLDB is not found relative to the
driver, the existing "integrated" REPL will be launched instead.*

If the user passes -lldb-repl and an LLDB is not found relative to the
driver, one from the user's PATH will be chosen (like the linker).

The user can also pass -integrated-repl to get the existing behavior.
"swift -frontend -repl" always uses the integrated REPL.

* Since LLDB's not quite ready to be the REPL yet, "swift -repl" still
invokes the integrated REPL. "swift -repl -experimental-prefer-lldb" tests
the new behavior; this option will become the default (and the flag removed)
in <rdar://problem/16776719>.

<rdar://problem/16776705>

Swift SVN r17134