[WebAssembly] Store function index rather than table index in TABLE_INDEX relocations

Relocations of type R_WEBASSEMBLY_TABLE_INDEX represent places
where the table index for a given function is needed.  While the
value stored in this location is a table index, the index in
the relocation entry itself is a function index (the index of
the function which is to be called indirectly).

This is how is was spec'd originally but the LLVM implementation
didn't do this.  This makes things a little simpler in the linker
since the table in the input file can essentially be ignored that
the output table can be created purely based on these relocations.

Patch by Nicholas Wilson!

Differential Revision: https://reviews.llvm.org/D42080

llvm-svn: 323165