[WebAssembly] Use LEB encoding for value types

[WebAssembly] Use LEB encoding for value types

Previously we were using the encoded LEB hex values
for the value types.  This change uses the decoded
negative value and the LEB encoder to write them out.

Differential Revision: https://reviews.llvm.org/D30847

Patch by Sam Clegg

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297777 91177308-0d34-0410-b5e6-96231b3b80d8