[Clang] Fix CXXRewrittenBinaryOperator::getDecomposedForm to handle case when spaceship operator returns comparison category by reference

Currently CXXRewrittenBinaryOperator::getDecomposedForm(...) may crash if the
spaceship operator returns a comparison category by reference. This because
IgnoreImplicitAsWritten() does not look through CXXConstructExpr. The fix is to
use IgnoreUnlessSpelledInSource() which will look though CXXConstructExpr.

This fixes: #64162

Author: shafik

clang/lib/AST/ExprCXX.cpp

@@ -111,7 +111,7 @@ CXXRewrittenBinaryOperator::getDecomposedForm() const {
     return Result;
 
   // Otherwise, we expect a <=> to now be on the LHS.
-  E = Result.LHS->IgnoreImplicitAsWritten();
+  E = Result.LHS->IgnoreUnlessSpelledInSource();
   if (auto *BO = dyn_cast<BinaryOperator>(E)) {
     assert(BO->getOpcode() == BO_Cmp);
     Result.LHS = BO->getLHS();

clang/test/SemaCXX/compare-cxx2a.cpp

@@ -479,3 +479,26 @@ void DoSomething() {
                                                   // expected-note  {{undefined function 'operator++' cannot be used in a constant expression}}
 }
 }
+
+namespace GH64162 {
+struct S {
+    const std::strong_ordering& operator<=>(const S&) const = default;
+};
+bool test(S s) {
+    return s < s; // We expect this not to crash anymore
+}
+
+// Following example never crashed but worth adding in because it is related
+struct A {};
+bool operator<(A, int);
+
+struct B {
+    operator A();
+};
+
+struct C {
+    B operator<=>(C);
+};
+
+bool f(C c) { return c < c; }
+}