[WebAssembly] Change indices types to unsined int (NFC)

Summary:
This changes int types to unsigned int in a few places: function indices
and `wasm::Valtype` (which is unsigend int enum).  Currently these
values cannot have negative values anyway, so this should not be a
functional change for now.

Reviewers: sbc100

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

Differential Revision: https://reviews.llvm.org/D54044

llvm-svn: 346031