[cmake] Copy system debugserver from the right place when only CommandLineTools

are installed

Instead of building debugserver when building lldb, I'd rather pass
LLDB_CODESIGN_IDENTITY="" to cmake and use the one already on my system.
However, on one of my machines I only have the CommandLineTools installed, and
so the hardcoded path to the system debugserver does not work for me.
Additionally, we should verify the LLDB framework exists on the machine before
trying to set the path to debugserver. This allows us to warn the user at
configure time that a system debugserver can't be found if they choose not to
build it themselves.

Patch by Alex Langford!

Differential Revision: https://reviews.llvm.org/D44507

llvm-svn: 327691