[NFC] WebAssembly build break #2

Summary:
Same as r332530, move WasmSymbol::dump to an implementation file to avoid linker
issues when the dump function is seen in the header, doesn't get eliminated, and
then linking fails because of the missing dependency.

<rdar://problem/40258137>

Reviewers: sbc100, ncw, paquette, vsk, dschuff

Subscribers: jgravelle-google, aheejin, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D46985

llvm-svn: 332542