When calling "settings set target.source-map <old-path> <new-path>", make sure...

When calling "settings set target.source-map <old-path> <new-path>", make sure that <new-path> exists before accepting it as a remapping.

We had some clients that had added old source paths remappings to their .lldbinit files and they were causing trouble at a later date. This fix should help mitigate these issues.

<rdar://problem/26358860>

llvm-svn: 274948