[WebAssembly] Encode numbers in ULEB128 in event section

The number of events and the type index should be encoded in ULEB128,
but they were incorrctly encoded in LEB128. The smallest number with
which its LEB128 and ULEB128 encodings are different is 64.
There's no way we can generate 64 events in the C++ toolchain
implementation so we can't test that, but the attached test tests when
the type index is 64.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D99627