[4.1][CursorInfo] Fix crash on instance variables used directly in if or for when declared in a generic context

[nathawes]

CCC Info

• Radar: rdar://problem/36871908
• Explanation:
  Invoking the cursor info request on a instance variable used directly in an if condition or for in clause would crash SourceKit if the instance variable was in a generic context. E.g:

class SomeType<T> {
  let test: Bool = false
  let items: [Int] = []
  func foo() {
    if test {} // crashes on test
    for in items {} // crashes on items
  }
}

We were getting an incorrect containing type for the instance variable (Bool rather than SomeType<T>) as our logic for matching cursor locations in MemberRefExprs was based on the source location of the . which isn't present in this case.

• Risk: Low. The change affects sourcekitd cursor info request when invoked on MemberRefExprs.
• Scope of issue: Jump-to-definition, quick help and refactoring were crashing sourcekit on these references
• Origination: Crash reports
• Reviewed by: Xi Ge
• Testing: Added regression test

PR for master: #14171
PR for swift-5.0-branch: #14189

[nkcsgexi]

LGTM!

[nathawes]

@swift-ci please test