[cling] Store enum values as data members to fix cppyy enum tests

This fixes the failing python enum tests like:
- `roottest-python-cpp-cpp`
- `roottest-python-cmdLineUtils-ROOT_8197`

Also remove `R__BYTESWAP` which should not be needed anymore.

The leak was fixed with:
llvm/llvm-project#78311

This caused our tests to fail as they relied on the
previous behavior.

Author: devajithvs

core/metacling/src/TClingDataMemberInfo.cxx

@@ -397,16 +397,17 @@ Longptr_t TClingDataMemberInfo::Offset()
    // clang there is misbehaviour in MangleContext::shouldMangleDeclName.
    // enum constants are essentially numbers and don't get addresses. However
    // ROOT expects the address to the enum constant initializer to be returned.
-   else if (const EnumConstantDecl *ECD = dyn_cast<EnumConstantDecl>(D))
+   else if (const EnumConstantDecl *ECD = dyn_cast<EnumConstantDecl>(D)) {
       // The raw data is stored as a long long, so we need to find the 'long'
       // part.
-#ifdef R__BYTESWAP
-      // In this case at the beginning.
-      return reinterpret_cast<Longptr_t>(ECD->getInitVal().getRawData());
-#else
-      // In this case in the second part.
-      return reinterpret_cast<Longptr_t>(((char*)ECD->getInitVal().getRawData())+sizeof(Longptr_t) );
-#endif
+
+      // The memory leak for `EnumConstantDecl` was fixed in:
+      // https://github.com/llvm/llvm-project/pull/78311
+      // We were relying on the leak to provide the address for EnumConstantDecl.
+      // Now store the data value as a member instead.
+      fEnumValue = ECD->getInitVal().getExtValue();
+      return reinterpret_cast<Longptr_t>(&fEnumValue);
+   }
    return -1L;
 }
 

core/metacling/src/TClingDataMemberInfo.h

@@ -77,6 +77,7 @@ class TClingDataMemberInfo final : public TClingDeclInfo {
    TClingDataMemberIter   fIter; // Current decl.
    std::string            fTitle; // The meta info for the member.
    bool                   fFirstTime = true; // We need to skip the first increment to support the cint Next() semantics.
+   int64_t                fEnumValue; // Special case to handle enums
 
    mutable std::string fIoType;
    mutable std::string fIoName;