[WebAssembly] Split WebAssemblyUtils to fix library layering for MC tools.

WebAssemblyUtils depends on CodeGen which depends on all middle end
optimization libraries.

This component is used by WebAssembly's AsmParser, Disassembler, and
MCTargetDesc libraries. Because of this, any MC layer tool built with
WebAssembly support includes a larger portion of LLVM than it should.

To fix this I've created an MC only version of WebAssemblyTypeUtilities.cpp
in MCTargetDesc to be used by the MC components.

This shrinks llvm-objdump and llvm-mc on my local release+asserts
build by 5-6 MB.

Reviewed By: MaskRay, aheejin

Differential Revision: https://reviews.llvm.org/D144354