[WebAssembly] Make segment/size/type directives optional in asm

Summary:
These were "boilerplate" that repeated information already present
in .functype and end_function, that needed to be repeated to Please
the particular way our object writing works, and missing them would
generate errors.

Instead, we generate the information for these automatically so the
user can concern itself with writing more canonical wasm functions
that always work as expected.

Reviewers: dschuff, sbc100

Subscribers: jgravelle-google, aheejin, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D57546

llvm-svn: 353067