[lldb] Have crashlog warn when remapped paths are inaccessible.

[lldb] Have crashlog warn when remapped paths are inaccessible.

It can be tricky to troubleshoot why the crashlog script can't show
inline sources. The two most common causes are that we couldn't find the
dSYM or, if we find the dSYM, that the path remapping included in the
dSYMForUUID output isn't accessible. The former is already easy to
diagnose, but the latter is harder because you'd have to manually invoke
dsymForUUID on the UUID and check the remapped path. This patch
automates that process and prints a warning if the remapped path doesn't
exist or is not accessible.

Differential revision: https://reviews.llvm.org/D152886