[clang][bytecode] Don't diagnose defined functions that will have a body

But don't have one, yet. That happens for class methods, which are
"defined" but have no body, hence they willHaveBody.

Fixes #164995

Author: tbaederr

clang/lib/AST/ByteCode/Interp.cpp

@@ -994,10 +994,12 @@ static bool CheckCallable(InterpState &S, CodePtr OpPC, const Function *F) {
           S.checkingPotentialConstantExpression())
         return false;
 
-      // If the declaration is defined, declared 'constexpr' _and_ has a body,
-      // the below diagnostic doesn't add anything useful.
+      // If the declaration is defined, declared 'constexpr' _and_
+      // has (or will have) a body, the below diagnostic doesn't add anything
+      // useful.
       if (DiagDecl->isDefined() && DiagDecl->isConstexpr() &&
-          DiagDecl->hasBody())
+          (DiagDecl->doesThisDeclarationHaveABody() ||
+           DiagDecl->willHaveBody()))
         return false;
 
       S.FFDiag(S.Current->getLocation(OpPC),

clang/test/AST/ByteCode/records.cpp

@@ -1861,3 +1861,12 @@ namespace PrimitiveInitializedByInitList {
   } c{ 17 };
   static_assert(c.b == 17, "");
 }
+
+namespace MethodWillHaveBody {
+  class A {
+  public:
+    static constexpr int get_value2() { return 1 + get_value(); }
+    static constexpr int get_value() { return 1; }
+  };
+  static_assert(A::get_value2() == 2, "");
+}