Finalize IDE support for `??=`

[333fred]

There are likely a few more places in the IDE layer to update for ??= support after it's merged to dev16.0.x, including:
UnwrapCompoundAssignment: http://source.roslyn.io/#Microsoft.CodeAnalysis.Features/ReplacePropertyWithMethods/AbstractReplacePropertyWithMethodsService.cs,6d51fab341c857cd,references
IsCompoundAssignment: http://source.roslyn.io/#Microsoft.CodeAnalysis.CSharp.Workspaces/Extensions/SyntaxNodeExtensions.cs,efadba0caa88db20,references
GetOperatorPrecedence: http://source.roslyn.io/#Microsoft.CodeAnalysis.CSharp.Workspaces/Extensions/ExpressionSyntaxExtensions.cs,f23f53d9b91d5532,references

[CyrusNajmabadi]

Have you looked at IsAnyAssignmentXXX and IsAssignmentXXX helpers? we should walk backwards from those to see what is affected. Also, can we have some type inference tests? i.e. if i have:

x ??= <-- what IDE type is inferred here.

This matters for things like completoin preselection (i.e. if 'x' is an enum type), as well as code-gen for things like x ??= NewMethod(); <-- gen method here

[jinujoseph]

cc @jasonmalinowski

[CyrusNajmabadi]

This will also affect: #29919

[333fred]

Manual Testing Notes:

1. Formatting - Verify spacing around, no spacing inside
2. Indentation - Typing Enter in an unfinished statement indents the next line
3. GoToDef/Impl/FAR - Verify doesn't break
4. Find References on LHS shows as read/write /cc @mavasani
5. Colorization - verify is colored as punctuation
6. Extract Method/Introduce Variable - Verify nothing is offered
7. Inline Temp - Verify nested cases a ??= (b ??= 1) // b is inlined
8. Convert to compound - do not attempt to convert a = a ?? throw null to a ??= throw null // Throw expressions not allowed /cc @CyrusNajmabadi
9. Quick Info - Verify nothing shown
10. Ilspy - Verify something sensical is spit out
11. EE - What happens in immediate window
12. Breakpoint spans - Verify it makes sense

[CyrusNajmabadi]

1. Need completion to show up on the right side if this is an enum type.
2. if someone is generating a method on the left or right side, it should figure out the right type.