[WebAssembly] Fix CFI index to account for padding nullptr function

The WebAssembly linker now creates a dummy function at index 0 to
prevent miscomparisons with the NULL pointer, see
https://github.com/WebAssembly/binaryen/pull/658. Thanks to pcc for
pointing out this problem!

Patch by Dominic Chen

Differential Revision: https://reviews.llvm.org/D23137

llvm-svn: 278073