[NFC] WebAssembly build fix

Summary:
r332305 added a use of llvm::wasm::toString in llvm::object::WasmSymbol::print,
which is in a header file. It also moves toString to BinaryFormat. This has the
unintended side-effect that any inclusion of Object/Wasm.h now relies on
toString, and needs to required_libraries = BinaryFormat. Thankfully most builds
don't fail with this because print just isn't used and gets eliminated, dropping
the required dependency in the process. Not all builds are so lucky.

Fix this issue by moving print to the corresponding .cpp file.

<rdar://problem/40258137>

Reviewers: sbc100, ncw, paquette

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

Differential Revision: https://reviews.llvm.org/D46977

llvm-svn: 332530