Set '.declarations' on a property of a homomorphic mapped type

[ghost]

Fixes #15821

[sandersn]

The change looks good but I'll wait to approve it until after the test failures are fixed to make sure there aren't any big changes that result.

[mhegazy]

The test failures need another look.

[ghost]

The bug in findAllRefsMappedType.ts was due to getSymbolScope not recognizing the mapped type property as a property. (Previously we would see symbol.declarations === undefined and immediately give up on getting a scope.) I changed it to always return undefined as soon as it sees that a symbol is a property or method -- I don't see why this wasn't done before.

[sandersn]

I'm not fluent with findAllRefs, so there may be an obvious answer: if a property or method is public, wouldn't it still need to be part of an exported type to be visible anywhere?

[ghost]

True, but that would be hard to detect. getSymbolScope definitely isn't looking for that right now.

[sandersn]

OK, that's what I needed to know. I wasn't sure of getSymbolScope's purpose.

[mhegazy]

getSymbolScope is just an optimization, so we only want to use it if we are sure the symbol will never escape this scope; otherwise we fall back to our normal exhaustive search. so it is ok for it to not handle all patterns.