Add test

Author: DavidGoldman

test/DebugInfo/debug_prefix_map.swift

@@ -1,7 +1,12 @@
-// RUN: %swiftc_driver -g -debug-prefix-map %/S=/var/empty %/s -emit-ir -o - | %FileCheck %s
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module-path %t/Globals.swiftmodule %S/Globals.swift
+// RUN: %swiftc_driver -g -debug-prefix-map %/S=/var/empty -debug-prefix-map %t=/var/empty %/s -I %t -emit-ir -o - | %FileCheck %s
+
+import Globals
 
 func square(_ n: Int) -> Int {
   return n * n
 }
 
 // CHECK: !DIFile(filename: "/var/empty/debug_prefix_map.swift"
+// CHECK: !DIModule(scope: null, name: "Globals", includePath: "/var/empty/Globals.swiftmodule"