Improve error handling in ObjectFileWasm by using helpers that wrap
their result in an llvm::Expected. The helper to read a Wasm string now
return an Expected<std::string> and I created a helper to parse 32-bit
ULEBs that returns an Expected<uint32_t>.

(cherry picked from commit 7cd6179)

My original implementation for parsing Wasm segments was wrong in two
related ways. I had a bug in calculating the file vm address and I
didn't fully understand the difference between active and passive
segments and how that impacted their file vm address.

With this PR, we now support parsing init expressions for active
segments, rather than just skipping over them. This is necessary to
determine where they get loaded.

Similar to llvm-objdump, we currently only support simple opcodes (i.e.
constants). We also currently do not support active segments that use a
non-zero memory index. However this covers all segments for a
non-trivial Swift binary compiled to Wasm.

(cherry picked from commit aadc708)