[Clang] Fix createConstexprUnknownAPValues to use zero offset when ceating APValue (#124478)

When implmenting P2280R4 here:
llvm/llvm-project#95474

When creating the APValue to store and constexprUnknown value I used an
offset of CharUnits::One() but it should have been CharUnits::Zero().

This change just adjusts that value.

Author: shafik

clang/lib/AST/ExprConstant.cpp

@@ -1961,7 +1961,7 @@ APValue &
 CallStackFrame::createConstexprUnknownAPValues(const VarDecl *Key,
                                                APValue::LValueBase Base) {
   APValue &Result = ConstexprUnknownAPValues[MapKeyTy(Key, Base.getVersion())];
-  Result = APValue(Base, CharUnits::One(), APValue::ConstexprUnknown{});
+  Result = APValue(Base, CharUnits::Zero(), APValue::ConstexprUnknown{});
 
   return Result;
 }