This adds support for serialization of `WasmEHFuncInfo`, in the form of
<Source BB Number, Unwind destination BB number>. To make YAML mapping
work, we needed to make a copy of the existing `SrcToUnwindDest` map
within `yaml::WebAssemblyMachineFunctionInfo`.
It was hard to add EH MIR tests for CFGStackify because `WasmEHFuncInfo`
could not be read from test MIR files. This adds the serialization
support for that to make EH MIR tests easier.
Reviewed By: dschuff
Differential Revision: https://reviews.llvm.org/D97174
51fb5bf4d6