x/typesutil:modify info.Overloads to point to the overload decl

[luoliwoshang]

modify info.Overloads to point to the overload decl instead point to overload member

type Info struct {
        ...
    // Overloads maps identifiers to the overload decl object.
    Overloads map[*ast.Ident][]types.Object
}

and modify the use of info.Overload in tools in this PR https://github.com/goplus/tools/pull/270

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.31%. Comparing base (5f7a7cd) to head (b0909d3).

:exclamation: Current head b0909d3 differs from pull request most recent head 967e9bb. Consider uploading reports for the commit 967e9bb to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1872 +/- ## ======================================= Coverage 93.31% 93.31% ======================================= Files 42 42 Lines 11663 11670 +7 ======================================= + Hits 10883 10890 +7 Misses 623 623 Partials 157 157 ```

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