[Dexter] Allow DexUnreachable in supplementary .dex files

DexUnreachable is a useful tool for specifying that lines shouldn't be
stepped on. Right now they have to be placed in the source file; lets allow
them to be placed instead in a detached .dex file, by adding on_line and
line-range keyword arguments to the command.

Differential Revision: https://reviews.llvm.org/D115449