[Sema] Downgrade to a warning public imports of an underlying private module

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -1769,10 +1769,15 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
                               "@_implementationOnly ");
         }
 
-        static bool treatAsError = getenv("ENABLE_PUBLIC_IMPORT_OF_PRIVATE_AS_ERROR");
 #ifndef NDEBUG
-        treatAsError = true;
+        static bool enableTreatAsError = true;
+#else
+        static bool enableTreatAsError = getenv("ENABLE_PUBLIC_IMPORT_OF_PRIVATE_AS_ERROR");
 #endif
+
+        bool isImportOfUnderlying = importer->getName() == target->getName();
+        bool treatAsError = enableTreatAsError &&
+                            !isImportOfUnderlying;
         if (!treatAsError)
           inFlight.limitBehavior(DiagnosticBehavior::Warning);
       }

test/Sema/Inputs/public-private-sdk/System/Library/PrivateFrameworks/MainLib.framework/Headers/MainLib.h

@@ -0,0 +1 @@
+void bar() {}

test/Sema/Inputs/public-private-sdk/System/Library/PrivateFrameworks/MainLib.framework/Modules/module.modulemap

@@ -0,0 +1,6 @@
+framework module MainLib {
+  umbrella header "MainLib.h"
+
+  export *
+  module * { export * }
+}

test/Sema/implementation-only-import-suggestion-as-error.swift

@@ -17,12 +17,13 @@
 // RUN: env ENABLE_PUBLIC_IMPORT_OF_PRIVATE_AS_ERROR=1 \
 // RUN:    %target-swift-frontend -typecheck -sdk %t/sdk -module-cache-path %t %s \
 // RUN:   -F %t/sdk/System/Library/PrivateFrameworks/ \
-// RUN:   -library-level api -verify
+// RUN:   -library-level api -verify -module-name MainLib
 
 import PublicSwift
-import PrivateSwift // expected-error{{private module 'PrivateSwift' is imported publicly from the public module 'main'}}
+import PrivateSwift // expected-error{{private module 'PrivateSwift' is imported publicly from the public module 'MainLib'}}
 
 import PublicClang
-import PublicClang_Private // expected-error{{private module 'PublicClang_Private' is imported publicly from the public module 'main'}}
-import FullyPrivateClang // expected-error{{private module 'FullyPrivateClang' is imported publicly from the public module 'main'}}
-import main // expected-warning{{'implementation-only-import-suggestion-as-error.swift' is part of module 'main'; ignoring import}}
+import PublicClang_Private // expected-error{{private module 'PublicClang_Private' is imported publicly from the public module 'MainLib'}}
+import FullyPrivateClang // expected-error{{private module 'FullyPrivateClang' is imported publicly from the public module 'MainLib'}}
+
+@_exported import MainLib // expected-warning{{private module 'MainLib' is imported publicly from the public module 'MainLib'}}

test/Sema/implementation-only-import-suggestion.swift

@@ -14,33 +14,33 @@
 /// Expect warnings when building a public client.
 // RUN: %target-swift-frontend -typecheck -sdk %t/sdk -module-cache-path %t %s \
 // RUN:   -F %t/sdk/System/Library/PrivateFrameworks/ \
-// RUN:   -library-level api -verify -D PUBLIC_IMPORTS
+// RUN:   -library-level api -verify -D PUBLIC_IMPORTS -module-name MainLib
 
 /// Expect no warnings when building an SPI client.
 // RUN: %target-swift-frontend -typecheck -sdk %t/sdk -module-cache-path %t %s \
 // RUN:   -F %t/sdk/System/Library/PrivateFrameworks/ \
-// RUN:   -library-level spi -D PUBLIC_IMPORTS
+// RUN:   -library-level spi -D PUBLIC_IMPORTS -module-name MainLib
 
 /// The driver should also accept the flag and pass it along.
 // RUN: %target-swiftc_driver -typecheck -sdk %t/sdk -module-cache-path %t %s \
 // RUN:   -F %t/sdk/System/Library/PrivateFrameworks/ \
-// RUN:   -library-level spi -D PUBLIC_IMPORTS
+// RUN:   -library-level spi -D PUBLIC_IMPORTS -module-name MainLib
 
 /// Expect no warnings when building a client with some other library level.
 // RUN: %target-swift-frontend -typecheck -sdk %t/sdk -module-cache-path %t %s \
 // RUN:   -F %t/sdk/System/Library/PrivateFrameworks/ \
-// RUN:   -D PUBLIC_IMPORTS
+// RUN:   -D PUBLIC_IMPORTS -module-name MainLib
 // RUN: %target-swift-frontend -typecheck -sdk %t/sdk -module-cache-path %t %s \
 // RUN:   -F %t/sdk/System/Library/PrivateFrameworks/ \
-// RUN:   -library-level other -D PUBLIC_IMPORTS
+// RUN:   -library-level other -D PUBLIC_IMPORTS -module-name MainLib
 #if PUBLIC_IMPORTS
 import PublicSwift
-import PrivateSwift // expected-error{{private module 'PrivateSwift' is imported publicly from the public module 'main'}}
+import PrivateSwift // expected-error{{private module 'PrivateSwift' is imported publicly from the public module 'MainLib'}}
 
 import PublicClang
-import PublicClang_Private // expected-error{{private module 'PublicClang_Private' is imported publicly from the public module 'main'}}
-import FullyPrivateClang // expected-error{{private module 'FullyPrivateClang' is imported publicly from the public module 'main'}}
-import main // expected-warning{{'implementation-only-import-suggestion.swift' is part of module 'main'; ignoring import}}
+import PublicClang_Private // expected-error{{private module 'PublicClang_Private' is imported publicly from the public module 'MainLib'}}
+import FullyPrivateClang // expected-error{{private module 'FullyPrivateClang' is imported publicly from the public module 'MainLib'}}
+@_exported import MainLib // expected-warning{{private module 'MainLib' is imported publicly from the public module 'MainLib'}}
 
 /// Expect no warnings with implementation-only imports.
 // RUN: %target-swift-frontend -typecheck -sdk %t/sdk -module-cache-path %t %s \