[WebAssembly] Update MC for bulk memory

Summary:
Rename MemoryIndex to InitFlags and implement logic for determining
data segment layout in ObjectYAML and MC. Also adds a "passive" flag
for the .section assembler directive although this cannot be assembled
yet because the assembler does not support data sections.

Reviewers: sbc100, aardappel, aheejin, dschuff

Subscribers: jgravelle-google, hiraditya, sunfish, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57938

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354397 91177308-0d34-0410-b5e6-96231b3b80d8
31 files changed