[ClangImporter] Do not import enum when already imported via DeclContext (#85424)

If we try to import this in ObjC interop mode:

```objc
typedef CF_OPTIONS(uint32_t, MyFlags) {
  ...
} CF_SWIFT_NAME(MyCtx.Flags);

struct MyStruct {
  MyFlags flags;
  ...
} CF_SWIFT_NAME(MyCtx);
```

ClangImporter tries to import `MyCtx/MyStruct` before it imports
`MyFlags` (via `importDeclContextOf()`), which in turn tries to import
`MyFlags` again due to the `flags` field. The existing cycle-breaking
mechanism prevents us from looping infinitely, but leads us to import
two copies of the Swift `EnumDecl`, which can cause errors later during
CodeGen.

~~This patch adds an assertion to catch such issues earlier, and breaks
the cycle by checking the cache again.~~ This patch no longer does so
because that caused issues beyond the scope of this patch.

rdar://162317760

lib/ClangImporter/ImportDecl.cpp

@@ -1613,6 +1613,14 @@ namespace {
       if (!dc)
         return nullptr;
 
+      // It's possible that we already encountered and imported decl while
+      // importing its decl context. If we are able to find a cached result,
+      // use it to avoid making a duplicate imported decl.
+      auto alreadyImported =
+          Impl.ImportedDecls.find({decl->getCanonicalDecl(), getVersion()});
+      if (alreadyImported != Impl.ImportedDecls.end())
+        return alreadyImported->second;
+
       auto name = importedName.getBaseIdentifier(Impl.SwiftContext);
 
       // Create the enum declaration and record it.

test/ClangImporter/circular-import-as-member.swift

@@ -0,0 +1,37 @@
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend -typecheck -verify -I %t/Inputs -cxx-interoperability-mode=default -module-name main %t/program.swift
+// RUN: %target-swift-frontend -typecheck -verify -I %t/Inputs -cxx-interoperability-mode=off -module-name main %t/program.swift
+// REQUIRES: objc_interop
+
+//--- Inputs/module.modulemap
+module TheModule {
+    header "the-header.h"
+}
+
+//--- Inputs/the-header.h
+#pragma once
+
+#define CF_SWIFT_NAME(_name) __attribute__((swift_name(#_name)))
+
+#define __CF_OPTIONS_ATTRIBUTES __attribute__((flag_enum,enum_extensibility(open)))
+#if (__cplusplus)
+#define CF_OPTIONS(_type, _name) __attribute__((availability(swift,unavailable))) _type _name; enum __CF_OPTIONS_ATTRIBUTES : _name
+#else
+#define CF_OPTIONS(_type, _name) enum __CF_OPTIONS_ATTRIBUTES _name : _type _name; enum _name : _type
+#endif
+
+typedef CF_OPTIONS(unsigned, TheFlags) {
+  TheFlagsFoo = (1 << 1),
+  TheFlagsBar = (1 << 2)
+} CF_SWIFT_NAME(The.Flags);
+
+typedef TheFlags DaFlags;
+
+struct TheContext {
+  TheFlags flags;
+} CF_SWIFT_NAME(The);
+
+//--- program.swift
+import TheModule
+
+func f(_ _: DaFlags) {}