[clang][Interp] Handle defined functions without a body

[tbaeder]

Summary

Turns out that this happens when explicitly defaulting a constructor or destructor.

[tbaeder]

CI fails because other patches are missing, most likely https://reviews.llvm.org/D140668.

clang/lib/AST/Interp/ByteCodeEmitter.cpp

if (!Func) {
     }
 
     Func =
         P.createFunction(FuncDecl, ParamOffset, std::move(ParamTypes),
                          std::move(ParamDescriptors), HasThisPointer, HasRVO);
   }
 
   assert(Func);
-  if (!HasBody)
+  // For not-yet-defined functions, we only create a Function instance and
+  // compile their body later.
+  if (!FuncDecl->isDefined())
     return Func;
 
   // Compile the function body.
   if (!FuncDecl->isConstexpr() || !visitFunc(FuncDecl)) {
     // Return a dummy function if compilation failed.
     if (BailLocation)
       return llvm::make_error<ByteCodeGenError>(*BailLocation);
     else {

clang/test/AST/Interp/records.cpp

struct YetAnotherDerived : YetAnotherBase {
     constexpr bool doit() const { return Val == OtherVal; }
   };
 
   constexpr YetAnotherDerived Oops(0); // ref-error {{must be initialized by a constant expression}} \
                                        // ref-note {{constructor inherited from base class 'YetAnotherBase' cannot be used in a constant expression}} \
                                        // expected-error {{must be initialized by a constant expression}}
                                        // FIXME: Missing reason for rejection.
 };
+
+namespace EmptyCtor {
+  struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
+  constexpr piecewise_construct_t piecewise_construct =
+    piecewise_construct_t();
+};