[ClangImporter] Protect against re-entrant bridging header loading (swiftlang#27045)

If, while loading a bridging header, we pick up a Clang module that
claims to have an overlay Swift module, and that Swift module turns
out to have a bridging header, we can end up reallocating the array
of modules to process while we're looping over it. Be defensive
against this occurrence.

This just fixes a crash; it does not at all solve the problem of this
being broken in several ways:

- Accidentally naming your module the same as a system module shadows
  the latter (if the system module is a Swift module) or *makes your
  module into an overlay* (if the system module is a Clang module).

- Bridging headers are only officially supported on executable targets
  and unit tests, but this isn't really enforced.

- Implicit inclusion of a bridging header *when you import a Swift
  module* is a hack to begin with, and a hack that worsens when the
  main module also has a bridging header. (All the bridging headers
  get folded together into the "same" module, which leads to more
  visibility than desired as well as cycles in the import graph.)

- Combining all of these can result in some pretty bizarre behavior.

rdar://problem/54581756

Author: jrose-apple

lib/ClangImporter/ClangImporter.cpp

@@ -1738,8 +1738,16 @@ void ClangImporter::Implementation::handleDeferredImports()
     ImportedHeaderExports.push_back(R.getSubmodule(ID));
   }
   PCHImportedSubmodules.clear();
-  for (const clang::Module *M : ImportedHeaderExports)
-    (void)finishLoadingClangModule(M, /*preferOverlay=*/true);
+
+  // Avoid a for-in loop because in unusual situations we can end up pulling in
+  // another bridging header while we finish loading the modules that are
+  // already here. This is a brittle situation but it's outside what's
+  // officially supported with bridging headers: app targets and unit tests
+  // only. Unfortunately that's not enforced.
+  for (size_t i = 0; i < ImportedHeaderExports.size(); ++i) {
+    (void)finishLoadingClangModule(ImportedHeaderExports[i],
+                                   /*preferOverlay=*/true);
+  }
 }
 
 ModuleDecl *ClangImporter::getImportedHeaderModule() const {

validation-test/ClangImporter/Inputs/bridging-header-reentrancy/A.h

@@ -0,0 +1,12 @@
+#include <CoincidentalNameCollision.h>
+#include <A.h>
+#include <B.h>
+#include <C.h>
+#include <D.h>
+#include <E.h>
+#include <F.h>
+#include <G.h>
+#include <H.h>
+#include <I.h>
+
+void appBridgingHeaderLoaded(void);

validation-test/ClangImporter/Inputs/bridging-header-reentrancy/App-Bridging-Header.h

@@ -0,0 +1 @@
+void CNCTest(void);