Fix handling of OO_Spaceship in DecodeOperatorCall

[sberg]

Summary

This seems to be a leftover from d30b23d6a54b3f0883914f3c2c2318a78edcbe67 "[c++2a] P0515R3: Support for overloaded operator<=>." (The corresponding llvm_unreachable in clang/lib/Sema/SemaOverload.cpp was e.g. fixed in 0683c0e68d31d18f6e4fedb270317844a4912882 "[C++2a] Implement operator<=> CodeGen and ExprConstant".)
It caused compilation of recent libstdc++ trunk <vector> to crash in -std=c++2a mode, hitting the llvm_unreachable.

[sberg]

Minimal reproducer is

template<typename> struct S;
template<typename T> void operator <=>(S<T>, S<T>);
template<typename T> void f(T x) { decltype(x <=> x) v; }

Added a corresponding test to clang/test/SemaCXX/cxx2a-three-way-comparison.cpp.

[sberg]

ping

[sberg]

ping^2

[rsmith]

An equivalent patch was landed as llvmorg-11-init-2132-gb96c6b65b93 to fix PR44786. Thanks!

clang/lib/AST/StmtProfile.cpp

     return Stmt::BinaryOperatorClass;
 
   case OO_Spaceship:
-    // FIXME: Update this once we support <=> expressions.
-    llvm_unreachable("<=> expressions not supported yet");
+    BinaryOp = BO_Cmp;
+    return Stmt::BinaryOperatorClass;
 
   case OO_AmpAmp:
     BinaryOp = BO_LAnd;

clang/test/SemaCXX/cxx2a-three-way-comparison.cpp

 };
 
 int &r = B().operator<=>(0);
+
+template<typename T> void f(T x) { decltype(x <=> x) v; }