Issue Description:

The following code yields different output on macOS (Foundation) vs Linux (SwiftFoundation):

NSString(string: "./").appendingPathComponent("/foo")

The output on macOS is: "./foo"
The output on Linux is: ".//foo"

Additional test script:

import Foundation
let a = NSString(string: "")
print(a.appendingPathComponent("foo"))
print(a.appendingPathComponent("/foo"))
print(a.appendingPathComponent("//foo"))
let b = NSString(string: "./")
print(b.appendingPathComponent("foo"))
print(b.appendingPathComponent("/foo"))
print(b.appendingPathComponent("//foo"))

Output on macOS:

foo
/foo
/foo
./foo
./foo
./foo

Output on Linux:

foo
/foo
//foo
./foo
.//foo
.///foo

The following is related: #5582.