[clang] fix merging of UsingShadowDecl

Previously, when deciding if two UsingShadowDecls where mergeable,
we would incorrectly only look for both pointing to the exact redecla
ration, whereas the correct thing is to look for declarations to the
same entity.

This problem has existed as far back as 2013, introduced in commit
fd8634a.

This problem could manifest itself as ODR check false positives
when importing modules.

Fixes: #80252

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -181,6 +181,9 @@ Bug Fixes to C++ Support
   and (`#79745 <https://github.com/llvm/llvm-project/issues/79745>`_)
 - Fix incorrect code generation caused by the object argument of ``static operator()`` and ``static operator[]`` calls not being evaluated.
   Fixes (`#67976 <https://github.com/llvm/llvm-project/issues/67976>`_)
+- Fix incorrect merging of modules which contain using declarations which shadow
+  other declarations. This could manifest as ODR checker false positives.
+  Fixes (`#80252 <https://github.com/llvm/llvm-project/issues/80252>`_)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/test/Modules/cxx20-decls.cppm

@@ -31,14 +31,12 @@ using xxx = baz::foo;
 // CHECK-NEXT: | `-BuiltinType 0x{{[^ ]*}} 'char'
 // CHECK-NEXT: |-UsingDecl 0x{{[^ ]*}} first 0x[[USING_ADDR:[^ ]*]] <{{.*}}> col:{{.*}} imported in A.<global> hidden baz::foo
 // CHECK-NEXT: | `-NestedNameSpecifier Namespace 0x[[BAZ_REDECL_ADDR]] 'baz'
-// FIXME: UsingShadowDecl should have been merged
-// CHECK-NOT:  `-UsingShadowDecl 0x{{[^ ]*}} prev 0x{{[^ ]*}} <{{.*}}> col:{{.*}} imported in A.<global> hidden implicit TypeAlias 0x[[ALIAS_REDECL_ADDR]] 'foo'
-// CHECK-NEXT: `-UsingShadowDecl 0x{{[^ ]*}} <{{.*}}> col:{{.*}} imported in A.<global> hidden implicit TypeAlias 0x[[ALIAS_REDECL_ADDR]] 'foo'
+// CHECK-NEXT: `-UsingShadowDecl 0x{{[^ ]*}} prev 0x[[SHADOW_ADDR:[^ ]*]] <{{.*}}> col:{{.*}} imported in A.<global> hidden implicit TypeAlias 0x[[ALIAS_REDECL_ADDR]] 'foo'
 
 // CHECK-LABEL: Dumping baz:
 // CHECK-NEXT: NamespaceDecl 0x[[BAZ_ADDR]] <{{.*}}> line:{{.*}} baz
 // CHECK-NEXT: |-TypeAliasDecl 0x[[ALIAS_ADDR]] <{{.*}}> col:{{.*}} referenced foo 'char'
 // CHECK-NEXT: | `-BuiltinType 0x{{[^ ]*}} 'char'
 // CHECK-NEXT: |-UsingDecl 0x[[USING_ADDR]] <{{.*}}> col:{{.*}} baz::foo
 // CHECK-NEXT: | `-NestedNameSpecifier Namespace 0x[[BAZ_ADDR]] 'baz'
-// CHECK-NEXT:  `-UsingShadowDecl 0x[[SHADOW_ADDR:[^ ]*]] <{{.*}}> col:{{.*}} implicit TypeAlias 0x[[ALIAS_ADDR]] 'foo'
+// CHECK-NEXT:  `-UsingShadowDecl 0x[[SHADOW_ADDR]] <{{.*}}> col:{{.*}} implicit TypeAlias 0x[[ALIAS_ADDR]] 'foo'