Diagnose if a SLEB128 is too large to fit in an int64_t.

Previously we'd hit UB due to an invalid left shift operand.

Also fix the WASM emitter to properly use SLEB128 encoding instead of
ULEB128 encoding for signed fields so that negative numbers don't
result in overly-large values that we can't read back any more.

In passing, don't diagnose a non-canonical ULEB128 that fits in a uint64_t but
has redundant trailing zero bytes.

Reviewed By: dblaikie, aardappel

Differential Revision: https://reviews.llvm.org/D95510