[WebAssembly] Allow for the creation of user-defined custom sections

This patch adds a way for users to create their own custom sections to
be added to wasm files. At the LLVM IR layer, they are defined through
the "wasm.custom_sections" named metadata. The expected use case for
this is bindings generators such as wasm-bindgen.

Patch by Dan Gohman

Differential Revision: https://reviews.llvm.org/D45297

llvm-svn: 329315