[Sema] Crash fix for nested type named identically to coding key

[gregomni]

Code to derive Codable was assuming any decl lookup of a coding key would be a VarDecl, now does the right thing in the presence of identically named other declarations (like nested types).

Resolves SR-6886.

[gregomni]

@swift-ci Please smoke test and merge.

[itaiferber]

I agree that crashing is bad, but how is this going to affect ill-formed code which used to crash but might now compile? i.e. we could be skipping over more "local" invalid decls which should produce diagnostics until we happen to find some VarDecl found in an outer scope, and synthesize based on that, no?

I'm trying to think of a concrete example, but it would be nice to verify with more extensive unit tests I think — at the very least some more deeply nested structs which might happen to find VarDecls from outer scopes even though decls closer by would be invalid and we should diagnose on.

[itaiferber]

To assuage my own concerns here — this only affects property lookup after CodingKeys validation is done. The following code fails validateCodingKeysEnum because that does direct property lookup via target->getStoredProperties:

struct Foo {
    struct Bar {
        struct Baz : Codable {
            private enum CodingKeys : CodingKey {
                case Bar
            }
        }
    }

    var Bar: String
}

The above produces

NestedStruct.swift:3:16: error: type 'Foo.Bar.Baz' does not conform to protocol 'Decodable'
        struct Baz : Codable {
               ^
Swift.Decodable:9:12: note: protocol requires initializer 'init(from:)' with type 'Decodable'
    public init(from decoder: Decoder) throws
           ^
NestedStruct.swift:5:22: note: CodingKey case 'Bar' does not match any stored properties
                case Bar
                     ^
NestedStruct.swift:3:16: error: type 'Foo.Bar.Baz' does not conform to protocol 'Encodable'
        struct Baz : Codable {
               ^
Swift.Encodable:12:17: note: protocol requires function 'encode(to:)' with type 'Encodable'
    public func encode(to encoder: Encoder) throws
                ^
NestedStruct.swift:5:22: note: CodingKey case 'Bar' does not match any stored properties
                case Bar
                     ^

I was afraid that case Bar would reach out to match var Bar from Foo, but that isn't the case here.

I still think this is worth adding a unit test for so we don't regress in the future, but this has already been merged.