[Support] Add a version of fs::make_absolute with a custom CWD.

This will be used soon from clang.

llvm-svn: 249309
diff --git a/llvm/unittests/Support/Path.cpp b/llvm/unittests/Support/Path.cpp
index a4d2ab1..f35682e 100644
--- a/llvm/unittests/Support/Path.cpp
+++ b/llvm/unittests/Support/Path.cpp
@@ -150,6 +150,10 @@
 
     path::native(*i, temp_store);
   }
+
+  SmallString<32> Relative("foo.cpp");
+  ASSERT_NO_ERROR(sys::fs::make_absolute("/root", Relative));
+  ASSERT_EQ("/root/foo.cpp", Relative);
 }
 
 TEST(Support, RelativePathIterator) {