[ClangImporter] Break cycle in IAM extensions

Under a set of circumstances that I cannot fully characterize, it was possible for `ClangImporter::Implementation::importDeclContextOf()` to end up recursing until stack overflow while trying to import a global as a member.

The cycle was caused by `addExtension()` using a newly-created extension’s lazy member loader to load its members before the extension had been added to ClangImporter’s `extensionPoints` table. Loading the members would try to import the IAM members again, which would cause `importDeclContextOf()` to try to create another extension to contain them, which `addExtension()` would force to load those members. Repeat until stack overflow.

I have fixed this bug by registering the extension in the `extensionPoints` map before its member loader is set. This ensures that if `importDeclContextOf()` gets called for any reason after that point, it will return the existing extension instead of creating a new one, avoiding the cycle.

Unfortunately, I was not able to figure out the exact conditions that were necessary to reproduce this bug, so I have not included a test. However, I have tested with the affected project locally and confirmed that this change lets it build. All existing tests pass.

Fixes rdar://120854030.

Author: beccadax

lib/ClangImporter/ImportDecl.cpp

@@ -9043,6 +9043,11 @@ ClangImporter::Implementation::importDeclContextOf(
                                      nominal->getDeclaredType());
   SwiftContext.evaluator.cacheOutput(ExtendedNominalRequest{ext},
                                      std::move(nominal));
+
+  // Record this extension so we can find it later. We do this early because
+  // once we've set the member loader, we don't know when the compiler will use
+  // it and end up back in this method.
+  extensionPoints[extensionKey] = ext;
   ext->setMemberLoader(this, reinterpret_cast<uintptr_t>(declSubmodule));
 
   if (auto protoDecl = ext->getExtendedProtocolDecl()) {
@@ -9052,8 +9057,6 @@ ClangImporter::Implementation::importDeclContextOf(
   // Add the extension to the nominal type.
   nominal->addExtension(ext);
 
-  // Record this extension so we can find it later.
-  extensionPoints[extensionKey] = ext;
   return ext;
 }