[Sema] Minimum access for protocol requirement members to 'fileprivate'.

[rintaro]

What's in this pull request?

CC: @jrose-apple
I'm not sure this is the right fix.

// ModA/File1.swift
private protocol P  {
  func privMethod() // This was implicitly `internal`.
}
public class C : P {
  public init() {}
  fileprivate func privMethod() {
    print("FOOBAR")
  }
}

C.privMethod() was visible from other file. (Because P.privMethod() was internal)

// ModA/File2.swift
extension C {
  public func callPrivMethod() {
    // This should be "error: use of unresolved identifier 'privMethod'",
    // but was not.
    privMethod()
  }
}

This compiles.
But, ends up with runtime error 'fatal error: call of deleted method'

// main.swift
C().callPrivMethod()

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[rintaro]

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[rintaro]

macOS: https://ci.swift.org/job/swift-PR-osx/3331/
Linux: https://ci.swift.org/job/swift-PR-linux/3522/

[jrose-apple]

SE-0025 says we shouldn't care if the member has a higher access, but this might be a good enough patch fix anyway.

(We have a similar problem on a larger scale in SR-2315, and I expect the fix for that will take care of this as well. But I think this is a reasonable thing to do anyway.)

[jrose-apple]

Both tests passed, force-merging.