Refactor wasm/WriterUtil.{cpp,h}.

Summary:
 - Makes code more in line with LLVM style (e.g. const char * -> StringRef)
 - Do not use formatv since we can construct message just by `+`
 - Replace some odd types such as `const StringRef` with more common type
 - Do not use default arguments if they are not necessary

Reviewers: sbc100

Subscribers: jfb, aheejin, llvm-commits, sunfish

Differential Revision: https://reviews.llvm.org/D43403

llvm-svn: 325383