[c++20] Implement P1185R2 (as modified by P2002R0).

For each defaulted operator<=> in a class that doesn't explicitly
declare any operator==, also inject a matching implicit defaulted
operator==.
diff --git a/clang/lib/Frontend/FrontendActions.cpp b/clang/lib/Frontend/FrontendActions.cpp
index 4d47c76..aeea63c 100644
--- a/clang/lib/Frontend/FrontendActions.cpp
+++ b/clang/lib/Frontend/FrontendActions.cpp
@@ -413,6 +413,8 @@
       return "ExceptionSpecInstantiation";
     case CodeSynthesisContext::DeclaringSpecialMember:
       return "DeclaringSpecialMember";
+    case CodeSynthesisContext::DeclaringImplicitEqualityComparison:
+      return "DeclaringImplicitEqualityComparison";
     case CodeSynthesisContext::DefiningSynthesizedFunction:
       return "DefiningSynthesizedFunction";
     case CodeSynthesisContext::RewritingOperatorAsSpaceship: