Simplify variant selection

[kyouko-taiga]

It seems like keeping track of mutation markers to create method/ subscript bundles isn't actually necessary for name resolution to pick the right candidates. Further access reification works better if we don't pre-filter the variants available on a free subscript bundle. For example:

subscript s(_ x: inout Int): Int {
  let { yield x }
  inout { yield &x }
}

public fun main() {
  var n = 0
  inout m = s[&n]
}

Here, name resolution should not filter out the inout variant only because the callee is not marked for mutation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.59%. Comparing base (f46bc7d) to head (be0f6b2).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1554      +/-   ##
==========================================
+ Coverage   87.58%   87.59%   +0.01%     
==========================================
  Files         372      372              
  Lines       22522    22511      -11     
==========================================
- Hits        19726    19719       -7     
+ Misses       2796     2792       -4     

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