* Apply -debug-prefix-map to isysroot
Previously the absolute path to the sysroot, which could differ across
machines in remote build scenarios, was included in the debug info of
object files:
```
!11 = distinct !DICompileUnit(language: DW_LANG_Swift, file: !12, producer: "Swift version 5.3-dev (LLVM 93caf26adb68d37, Swift 58b2ddc65afbd60)", isOptimized: false, runtimeVersion: 5, emissionKind: FullDebug, enums: !13, imports: !14, sysroot: "/Applications/Xcode-11.5.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk", sdk: "MacOSX.sdk")
```
With this change these are included in the remapping done by
`-debug-prefix-map`.
* Fix test
