[WebAssembly] Fixed relocation errors having no location.
Used to print:
<unknown>:0: error: Cannot represent a difference across sections
(the location was null).
err.s:20:3: error: Cannot represent a difference across sections
i32.const foo-bar ^
Note: I looked at adding a test for this, but I don't think it is
worth it. We're not testing error formatting in the Wasm backend :)
Reviewers: sbc100, jgravelle-google
Subscribers: dschuff, aheejin, sunfish, llvm-commits
Differential Revision: https://reviews.llvm.org/D65602