[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329315 91177308-0d34-0410-b5e6-96231b3b80d8