Windows-only compilation failure using `#sourceLocation`

[grynspan]

Description
Using #sourceLocation(file:line:) fails on Windows only with the latest nightly toolchain (as of 2023-01-02.)

Steps to reproduce

In a Swift file, write:

#sourceLocation(file: "example.swift", line: 123)

Then build.

Expected behavior
Build succeeds.

Environment

• Swift compiler version info compnerd.org Swift version 5.8-dev (LLVM ab856b0fa3177ee, Swift 71c62c0)
• Deployment target: x86_64-unknown-windows-msvc

[grynspan]

The following test target fails to build:

import XCTest

final class Test: XCTestCase {
  func testHuh() {
    #sourceLocation(file: "a.swift", line: 1)
  }
}```

[grynspan]

Dup of #57553