DebugInfo: Provide option for explicitly specifying the name of the DWP file

DebugInfo: Provide option for explicitly specifying the name of the DWP file

If you've archived the DWP file somewhere it's probably useful to be
able to just tell llvm-symbolizer where it is when you're symbolizing
stack traces from the binary.

This only provides a mechanism for specifying a single DWP file, good if
you're symbolizing a program with a single DWP file, but it's likely if
the program is dynamically linked that you might have a DWP for each
dynamic library - in which case this feature won't help (at least as
it's surfaced in llvm-symbolizer for now) - in theory it could be
extended to specify a collection of DWP files that could all be
consulted for split CU hash resolution.

llvm-svn: 309498