[lldb] Copy log files into diagnostic directory

This patch copies over log files to the diagnostic directory. The caveat
here is that this only works for logs that are redirected to a file. The
implementation piggybacks of the mapping kept by the debugger. The
advantage is that it's free until you generate the diagnostics, at which
point you only pay the price of copying over the file.

Differential revision: https://reviews.llvm.org/D135631