[WebAssembly] Rename methods in WasmEHFuncInfo (NFC)

This renames variable and method names in `WasmEHFuncInfo` class to be
simpler and clearer. For example, unwind destinations are EH pads by
definition so it doesn't necessarily need to be included in every method
name. Also I am planning to add the reverse mapping in a later CL,
something like `UnwindDestToSrc`, so this renaming will make meanings
clearer.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D97173