[WebAssebmly] Report undefined symbols correctly in objdump

Peviously we were reporting undefined symbol as being defined
by the IMPORT sections.

This change reports undefined symbols in the same that other
formats do, and also removes the need to store the section
with each symbol (since it can be derived from the symbol
type).

Differential Revision: https://reviews.llvm.org/D43101

llvm-svn: 324770