Fix indexing crasher with implicit objcImpl inits

[beccadax]

Implicit initializers are given a source location within the type they belong to. This works poorly for @objc @implementation classes, because the class they belong to is imported and so those SourceLocs are in a different source buffer from the extension they’re inside, breaking an invariant enforced by index-while-building features.

Fix these SourceLocs to come from the implementation context, so they’ll come from the extension for an objcImpl class and the type itself otherwise.

[beccadax]

@swift-ci please test

[beccadax]

@bnbarham Would especially appreciate your input, as I'm not very familiar with indexing.

[bnbarham]

Thanks @beccadax! Seems reasonable to me. My only comment would be that instead of printing the index symbols from swift-ide-test after already indexing using index-store-path, we could just print the record and check that instead.

And I'm assuming this is a similar crash to #78835, but where the buffer ID is actually valid (as we're currently indexing the Swift file, but getting a location outside it)?

[beccadax]

@swift-ci please test

[beccadax]

@bnbarham Yes, it's a very similar crash—just caused by seeing a source location belonging to e.g. buffer 42 when we're indexing buffer 13, rather than seeing a source location for 13 when we're indexing the nonexistent -1.