[WebAssembly] Use section index in relocation section header

Rather than referring to sections my their code, use the
absolute index of the target section within the module.

See https://github.com/WebAssembly/tool-conventions/issues/52

Differential Revision: https://reviews.llvm.org/D45980

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330749 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed