[WebAssembly] clang-tidy (NFC)

Summary:
This patch fixes clang-tidy warnings on wasm-only files.
The list of checks used is:
`-*,clang-diagnostic-*,llvm-*,misc-*,-misc-unused-parameters,readability-identifier-naming,modernize-*`
(LLVM's default .clang-tidy list is the same except it does not have
`modernize-*`.)

The list of fixes are:
- Variable names start with an uppercase letter
- Function names start with a lowercase letter
- Use `auto` when you use casts so the type is evident

Reviewers: sbc100

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

Differential Revision: https://reviews.llvm.org/D57499

llvm-svn: 353076