[WebAssembly] Always start table index at 1, even for relocatable output

Previously llvm was using 0 as the first table index for wasm object
files but now that has switched to 1 we can have the output of lld
do the same and simplify the code.

Patch by Nicholas Wilson!

Differential Revision: https://reviews.llvm.org/D42096

llvm-svn: 323378