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

Diagnostics of public imports of private modules are more likely to be
wrong about imports of the underlying module from a Swift overlay due to
project configuration/installation, or the fact that private overlays
are installed in the same folder as public ones. Let's always downgrade
these diagnostics to warnings to help landing the rest of the
diagnostics as errors.

rdar://87262431

Author: xymus

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 \