URL: Don't standardize relative file paths

[jrflat]

Resolves #790 (thanks @nicklockwood for filing!)

We should not standardize the relative path outside of the context of its base URL, such as when calculating the absolute .path(). This allows us to properly handle ".." segments at the beginning of the relative path, such as in

let baseURL = URL(filePath: "/docs/src/")
let result = URL(filePath: "../images/foo.png", relativeTo: baseURL)
XCTAssertEqual(result.path(), "/docs/images/foo.png")

Note that if the inputted path is absolute, e.g. "/../images/foo.png", we should not resolve against the base URL. I've added a small unit test to cover both cases.

[jrflat]

@swift-ci please test

[DmT021]

@jrflat @parkera Could it be the source of this error:

/home/build-user/swift-integration-tests/test-static-stdlib/test-static-stdlib.test:7:8: error: CHECK: expected string not found in input
CHECK: foo bar baz: file:/foo
       ^
<stdin>:1:1: note: scanning from here
foo bar baz: file:///foo
^

Input file: <stdin>
Check file: /home/build-user/swift-integration-tests/test-static-stdlib/test-static-stdlib.test

-dump-input=help explains the following input dump.

Input was:
<<<<<<
         1: foo bar baz: file:///foo 
check:7     X~~~~~~~~~~~~~~~~~~~~~~~~ error: no match found
>>>>>>

error: command failed with exit status: 1

I see it on CI in this build: https://ci.swift.org/job/swift-PR-Linux/16654/console