[5.0][CodeCompletion] Rework getOperatorCompletions()

[rintaro]

(Cherry-pick of #20632 to swft-5.0-branch)

In postfix completion, for operator completion, we do:

1. Type check the operand without applying it, but set the resolved
   type to the root of the expression.
2. For each possible operators:
   i. Build temporary binary/postfix expression
   ii. Perform type checking to see whether the operator is applicable

This could be very slow especially if the operand is complex.

• Introduce ReusePrecheckedType option to constraint system. With
  this option, CSGen respects pre-stored types in expressions and doesn't
  take its sub-expressions into account.

  • Improve type checking performance because type variables aren't
    generated for sub-expressions of LHS (45511835)
  • Guarantee that the operand is not modified by the type checker because
    expression walkers in CSGen doesn't walk into the operand.

• Introduce TypeChecker::findLHS() to find LHS for a infix operator from
  pre-folded expression. We used to foldSequence() temporary
  SequenceExpr and find 'CodeCompletionExpr' for each attempt.

  • No need to flatten folded expression after initial type-checking.
  • Save memory of temporary BinaryExpr which used to be allocated by
    foldSequence().
  • Improve accuracy of the completion. foldSequence() recovers invalid
    combination of operators by left associative manner (with
    diagnostics). This used to cause false-positive results. For instance,
    a == b <HERE> used to suggest == operator. findLHS() returns
    nullptr for such invalid combination.

rdar://problem/43008573
rdar://problem/45511835
https://bugs.swift.org/browse/SR-9061

[akyrtzi]

@swift-ci Please test