[ConstraintSystem] Fix non-determinism in diagnoseAmbiguity

[xedin]

Simplification of member locator would produce a base expression,
this is what we want for diagnostics but not for comparisons in
diagnoseAmbiguity because base expression is located at a different
depth which would lead to incorrect results if both reference and base
expression are ambiguous e.g. test[x].count if both [x] and count
are ambiguous than simplification of count would produce [x] which
is incorrect.

This is the test-case (already in the suite) that exibits this behavior:

func test_ambiguity_with_placeholders(pairs: [(rank: Int, count: Int)]) -> Bool {
  return pairs[<#^ARG^#>].count == 2
}

Here subscript would either return a tuple or ArraySlice and
count is ambiguous because both have it.

[xedin]

@swift-ci please smoke test