Add ability to set a break point on a constructor declared by a `union` declaration

[AlekseyTs]

Given a union declaration union U (int, bool);, for int and bool we create a constructor. For each of them a sequence point pointing to the corresponding type syntax is added at the end of the constructor's body (this is very similar to what we do for record constructors).

The idea is that a user should be able to set a break point for the constructors by using the corresponding type syntax. This doesn't work at the moment. While stepping, debugger stops at the location, but there is no way to set a break point at it

From @tmat:

Seems similar to records. Definitely non-trivial. Needs updates in multiple places in IDE (breakpoint placement and EnC).

[tmat]

Updates needed for breakpoint placement:

• BreakpointSpans.TryCreateSpanForNode

Updates needed for EnC to handle the breakpoint:

• A new declaration body definition needs to be added: https://github.com/dotnet/roslyn/tree/main/src/Features/CSharp/Portable/EditAndContinue/DeclarationBody

Updates need for EnC to handle unions in general:

• Labels for the union itself and for case types added to https://github.com/dotnet/roslyn/blob/main/src/Features/CSharp/Portable/EditAndContinue/SyntaxComparer.cs
• GetSymbolForEdit might need update
• Depending on the exact code gen for unions AddConstructorEdits and other places in the analyzer might need update
• Handle edits that add/remove/update case type (equivalent to adding/removing/updating constructor).