[lldb][COFF] Map symbols without base+complex type as 'Data' type

Both LLD and GNU ld write global/static variables to the COFF symbol
table with `IMAGE_SYM_TYPE_NULL` and `IMAGE_SYM_DTYPE_NULL` type. Map
these symbols as 'Data' type in the symtab to allow these symbols to be
used in expressions and printable.

Reviewed By: labath, DavidSpickett

Differential Revision: https://reviews.llvm.org/D134585