Issue Description:

I'm trying to do some string processing that looks something like this:

import Foundation

let lines = """
BuildVersion: "123"
ProductType: "SomeProduct1,2"
HWModelStr: "TotallyNotAP"
"""

for line in lines.split(separator: "\n", omittingEmptySubsequences: true) {
var x: String
if line.starts(with: "BuildVersion") {
x = String(line.split(separator: """)[1])
} else if line.starts(with: "ProductType") {
x = line.split(separator: """)[1]
.trimmingCharacters(in: CharacterSet.letters.inverted)
.lowercased()
} else if line.starts(with: "HWModelStr") {
let y = line.split(separator: """)[1]
if y.hasSuffix("AP") {
x = String(y.dropLast(2))
} else {
x = String👍
}
} else {
x = ""
}

print❌
}

Which works in Darwin land,
>>>
123
someproduct
TotallyNot

but on Linux I get errors like

error: terminated(1): /usr/bin/swift-build-tool -f /swift-execution/.build/debug.yaml main
ERROR at line 34, col 40: value of type 'Substring.SubSequence' (aka 'Substring') has no member 'trimmingCharacters'
x = line.split(separator: """)[1]
~~~~~~~~~~~~~~~~~~~~~~~~~~_^~
ERROR at line 39, col 12: value of type 'Substring.SubSequence' (aka 'Substring') has no member 'hasSuffix'
if y.hasSuffix("AP") {
^ ~~~~~~~~~

I would expect these functions to work here.