[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

llvm/lib/MC/WasmObjectWriter.cpp

@@ -338,7 +338,7 @@ struct WasmImport {
   StringRef ModuleName;
   StringRef FieldName;
   unsigned Kind;
-  uint32_t Type;
+  int32_t Type;
 };
 
 // A wasm function to be written into the function section.