Invalid nested extension hits AvailabilityScope verifier failure with `-library-level api`

[hamishknight]

The following hits an AvailabilityScope verification error when built with -library-level api:

public class C {
  public subscript() -> Int {
    get {
      extension C {}

Stack dump:
0.      Program arguments: /Users/hamish/src/swift-dev/build/Release/swift-macosx-arm64/bin/swift-frontend -typecheck /Users/hamish/src/swift-test-arena/main.swift -sdk /Users/hamish/src/MacOSX.sdk -debug-diagnostic-names -diagnostic-style=llvm -library-level api
1.      Swift version 6.2-dev (LLVM b3ce1bce68b3ae6, Swift 8b4740f0ccf6218)
2.      Compiling with effective version 5.10
3.      Abort: function verificationError at AvailabilityScope.cpp:531
|       child source range not contained
|       child:
|       (decl_implicit version=15.0 decl=extension.C src_range=[/Users/hamish/src/swift-test-arena/main.swift:1277:7 - line:1277:21])
|       parent:
|       (decl_implicit version=15.0 decl=_ src_range=[/Users/hamish/src/swift-test-arena/main.swift:1276:9 - line:1277:20]
|         (decl_implicit version=15.0 decl=extension.C src_range=[/Users/hamish/src/swift-test-arena/main.swift:1277:7 - line:1277:21]  ))
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  swift-frontend           0x00000001087ba020 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1  swift-frontend           0x00000001087b833c llvm::sys::RunSignalHandlers() + 112
2  swift-frontend           0x00000001087ba69c SignalHandler(int, __siginfo*, void*) + 324
3  libsystem_platform.dylib 0x0000000182672744 _sigtramp + 56
4  libsystem_pthread.dylib  0x00000001826688cc pthread_kill + 296
5  libsystem_c.dylib        0x000000018256f7ac abort + 124
6  swift-frontend           0x000000010442d684 _abortWithMessage(llvm::StringRef) + 172
7  swift-frontend           0x000000010442d748 _ABORT(char const*, int, char const*, llvm::StringRef) + 0
8  swift-frontend           0x00000001040f5000 swift::AvailabilityScope::verify(swift::AvailabilityScope const*, swift::ASTContext&) const + 516
9  swift-frontend           0x00000001040f4e40 swift::AvailabilityScope::verify(swift::AvailabilityScope const*, swift::ASTContext&) const + 68
10 swift-frontend           0x00000001040f4e40 swift::AvailabilityScope::verify(swift::AvailabilityScope const*, swift::ASTContext&) const + 68
11 swift-frontend           0x00000001040ab920 swift::verify(swift::SourceFile&) + 116
12 swift-frontend           0x000000010437ad74 swift::TypeCheckPrimaryFileRequest::cacheResult(std::__1::tuple<>) const + 76
13 swift-frontend           0x0000000103e45ee0 swift::performTypeChecking(swift::SourceFile&) + 92
14 swift-frontend           0x0000000102e60ed4 bool llvm::function_ref<bool (swift::SourceFile&)>::callback_fn<swift::CompilerInstance::performSema()::$_0>(long, swift::SourceFile&) + 16
15 swift-frontend           0x0000000102e580f8 swift::CompilerInstance::forEachFileToTypeCheck(llvm::function_ref<bool (swift::SourceFile&)>) + 164
16 swift-frontend           0x0000000102e58034 swift::CompilerInstance::performSema() + 76
17 swift-frontend           0x0000000102c18750 withSemanticAnalysis(swift::CompilerInstance&, swift::FrontendObserver*, llvm::function_ref<bool (swift::CompilerInstance&)>, bool) + 60
18 swift-frontend           0x0000000102c0e0a0 performCompile(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 700
19 swift-frontend           0x0000000102c0d89c swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2276
20 swift-frontend           0x00000001029d9f28 swift::mainEntry(int, char const**) + 3112
21 dyld                     0x00000001822b1c84 start + 7040