[WebAssembly] De-dup indirect function table.

Create the indirect function table based on symbols rather
than just duplicating the input entries.  This has the
effect of de-duplicating the table.

This is a followup to the equivalent change made for globals:
  https://reviews.llvm.org/D40859

Partially based on a patch by Nicholas Wilson:
  https://reviews.llvm.org/D40845

Differential Revision: https://reviews.llvm.org/D40989

llvm-svn: 320428