[lldb] Upgrade `GetIndexOfChildWithName` to use `llvm::Expected` (#136693)

This patch replaces the use of `UINT32_MAX` as the error return value of
`GetIndexOfChildWithName` with `llvm::Expected`.


# Tasks to do in another PR

1. Replace `CalculateNumChildrenIgnoringErrors` with
`CalculateNumChildren`. See [this
comment](https://github.com/llvm/llvm-project/pull/136693#discussion_r2056319358).
2. Update `lldb_private::formatters::ExtractIndexFromString` to use
`llvm::Expected`. See [this
comment](https://github.com/llvm/llvm-project/pull/136693#discussion_r2054217536).
3. Create a new class which carries both user and internal errors. See
[this
comment](https://github.com/llvm/llvm-project/pull/136693#discussion_r2056439608).

1 files changed, 3 insertions, 2 deletions

diff --git a/lldb/source/Plugins/Language/ObjC/NSException.cpp b/lldb/source/Plugins/Language/ObjC/NSException.cpp
index 67f3f1779e14..e27c7e0e97be 100644
--- a/lldb/source/Plugins/Language/ObjC/NSException.cpp
+++ b/lldb/source/Plugins/Language/ObjC/NSException.cpp
@@ -148,7 +148,7 @@ public:
                : lldb::ChildCacheState::eRefetch;
   }
 
-  size_t GetIndexOfChildWithName(ConstString name) override {
+  llvm::Expected<size_t> GetIndexOfChildWithName(ConstString name) override {
     // NSException has 4 members:
     //   NSString *name;
     //   NSString *reason;
@@ -162,7 +162,8 @@ public:
     if (name == g_reason) return 1;
     if (name == g_userInfo) return 2;
     if (name == g_reserved) return 3;
-    return UINT32_MAX;
+    return llvm::createStringError("Type has no child named '%s'",
+                                   name.AsCString());
   }
 
 private: