libclang: do not make error locations relative

This is done at a later time anyway, so doesn't need to be done in the
cgo package. In fact, _not_ doing it there makes it easier to print
correct relative packages.

testdata/errors/cgo.go

@@ -7,9 +7,6 @@ import "C"
 func main() {
 }
 
-// TODO: this error should be relative to the current directory (so cgo.go
-// instead of testdata/errors/cgo.go).
-
 // ERROR: # command-line-arguments
-// ERROR: testdata/errors/cgo.go:3:5: error: hello
-// ERROR: testdata/errors/cgo.go:4:4: error: expected identifier or '('
+// ERROR: cgo.go:3:5: error: hello
+// ERROR: cgo.go:4:4: error: expected identifier or '('