[CodeCompletion] Fix an issue that causes an iterator to be invalidated while iterating
In `SourceLookupCache::lookupVisibleDecls`, copy the top level values before iterating them.
If we have 'addinitstotoplevel' enabled, calling 'addConstructorCallsForType' can cause macros to get expanded, which can then cause new members ot get added to 'TopLevelValues', invalidating the current iterator.
I have not been able to reduce this to a test case that doesn’t rely on the `Observation` module in the SDK but here is the test case with which I was able to reproduce the issue very reliably.
```swift
import Foundation
import Observation
@Observable class MyObject {}
extension MyObject {}
// RUN: ~/sbin/sourcekitd-test \
// RUN: -req=complete.open \
// RUN: -req-opts=addinitstotoplevel=1 \
// RUN: -pos=8:1 \
// RUN: %s \
// RUN: -- \
// RUN: %s \
// RUN: -Xfrontend \
// RUN: -load-plugin-library \
// RUN: -Xfrontend \
// RUN: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/plugins/libObservationMacros.dylib \
// RUN: -sdk \
// RUN: /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.0.sdk
```
rdar://109202157
swift
0996342b - [CodeCompletion] Fix an issue that causes an iterator to be invalidated while iterating
