[SR-12323] Have `Decodable` synthesis ignore conformance of `let` members.

[swift-ci]

Previous ID	SR-12323
Radar	rdar://problem/60230616
Original Reporter	Lantua (JIRA User)
Type	Improvement

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | | |Labels | Improvement | |Assignee | None | |Priority | Medium | md5: d33e7d3a3e2499fac13d99906bfc8f7b

Issue Description:

Currently `Decodable` synthesis leave out `let` member, but it still require that those members conform to `Decodable`.

Maybe we can lift this restriction?

[beccadax]

@swift-ci create