[WebAssembly] Fix import type to be signed LEBs

This fix is a follow up a previous change with stored
value types as signed integers in memory.

In future, once the yaml<->wasm binary patche lands we
can add test coverage for this kind of thing.

Differential Revision: https://reviews.llvm.org/D31227

Patch by Sam Clegg

llvm-svn: 298612