Champion "Expression variables in initializers" (15.7)

- [X] Proposal added: [proposals/csharp-7.3/expression-variables-in-initializers.md](https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.3/expression-variables-in-initializers.md)
- [x] Discussed in LDM
- [x] Decision in LDM
- [ ] Finalized (done, rejected, inactive)
- [ ] Spec'ed

See also https://github.com/dotnet/roslyn/issues/16270

There are three contexts where out variable declarations and pattern matching variable declarations are not permitted today, in part because we did not decide what the scope should be. I propose we decide for each of these three contexts, and extend the language to permit declaration expressions in these contexts.

1. Constructor-initializers. The primary question is whether the scope of variables declared in a ctor-initializer extends to the constructor body or not. I propose that it should extend to the body.

2. Field initializers. The primary question is whether the scope of variables declared in the one *equals-value-clause* is in scope in subsequent *equals-value-clause*s of the same field declaration (when it declares more than one field). It is also possible to extend the scope to subsequent separate field declaration's initializers too, but I don't think that is likely to be approved by the LDM.

3. Query clauses. The primary question is what is the scope of variables declared in a query clause that is specified to be translated into the body of a lambda. One possible resolution is that its scope is just that expression of the clause.