[WebAssembly] MC: Refactor relocation handling

The change cleans up and unifies the handling of relocation
entries in WasmObjectWriter.  Type index relocation no longer
need to be handled separately.

The only externally visible change should be that type
index relocations are no longer grouped at the end.

Differential Revision: https://reviews.llvm.org/D33918

llvm-svn: 304816