Track .dwo/.dwp loading errors and notify user when viewing variables.

Track .dwo/.dwp loading errors and notify user when viewing variables.

When debugging using Fission (-gsplit-dwarf), we can sometimes have issues loading .dwo files if they are missing or if the path was relative and we were unable to locate the file. We can also skip loading due to DWO ID mismatch or if a .dwp file doesn't contain a matching .dwo file. Also .dwo files could be updated due to a recompile and if the user debugs an executable that was linked against the old .dwo file, it could fail to load the information.

This patch adds a m_dwo_error to DWARFUnit that can be get/set and will cause "frame variable" to show errors when there are .dwo/.dwp issues informing the user about the error.

Differential Revision: https://reviews.llvm.org/D134252