[lldb] Allow plugins to extend DWARF expression parsing for vendor extensions

Parsing DWARF expressions currently does not support DW_OPs that are vendor
extensions. With this change expression parsing calls into SymbolFileDWARF for
unknown opcodes, which is the semantically "closest" plugin that we have right
now. Plugins can then extend SymbolFileDWARF to add support for vendor
extensions.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D137247