[lldb][COFF] Improve info of symbols from export table

- Skip dummy/invalid export symbols.
- Make the export ordinal of export symbols visible when dumping the
  symtab.
- Stop setting the 'Debug' flag and set the 'External' flag instead to
  better match the meaning of export symbols.
- Try to guess the type (code vs data) of the symbol from section flags.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D134265