[LLDB] Impossible to inherit environment in REPL

[al45tair]

Passing -O “settings set target.inherit-env 1” to the REPL doesn’t cause it to inherit the environment. (Apparently the REPL process gets launched without dealing with the environment settings.)

(This is also rdar://137522456).

[compnerd]

CC: @fredriss - we should try to get this resolved in time for the next 6.0 dot release.