Currently if you evaluate for example "expression int" the user just sees nothing and has no way of knowing what
exactly could have gone wrong. This patch adds a -w flag to the expression command that causes compiler warnings
to be emitted even if the command succeeds. This was the agreed workaround to solve rdar://12788008.
The checkerror flag that was added to lldbtest was necessary as we usually don't check the error output of commands
when they succeed (but we have to do now).