[WebAssembly] Add WasmEHFuncInfo for unwind destination information

Summary:
Add WasmEHFuncInfo and routines to calculate and fill in this struct to
keep track of unwind destination information. This will be used in
other EH related passes.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, chrib, llvm-commits

Differential Revision: https://reviews.llvm.org/D48263

llvm-svn: 335005