[WebAssembly] TargetStreamer cleanup (NFC)

Summary:
- Unify mixed argument names (`Symbol` and `Sym`) to `Sym`
- Changed `MCSymbolWasm*` argument of `emit***` functions to `const
  MCSymbolWasm*`. It seems not very intuitive that emit function in the
  streamer modifies symbol contents.
- Moved empty function bodies to the header
- clang-format

Reviewers: aardappel, dschuff, sbc100

Subscribers: jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D55347

llvm-svn: 348816