Print a warning when stopped in a frame LLDB has no plugin for.

This patchs adds an optional warning that is printed when stopped at a
frame that was compiled in a source language that LLDB has no plugin
for.

The motivational use-case is debugging Swift code on Linux. When the
user accidentally invokes the system LLDB that was built without the
Swift plugin, it is very much non-obvious why debugging doesnt
work. This warning makes it easy to figure out what went wrong.

<rdar://problem/56986569>