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 goplus/tools#270

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

❗ 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

@@           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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