Skip XRef reconstruction for new-style XRef streams

XRef::constructXRef was invented to support old style XRef tables.
Sadly it won't work for XRef streams. If applied anyway, it corrupts our
existing XRef::entries array. Better skip reconstruction for the XRef-stream-case
in XRef::readXRefUntil (just like XRef::fetch already does).

Fixes #139.
3 jobs for skip-constructxref-if-xrefstream in 2 minutes and 42 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #26253
build

00:02:42

passed #26254
build_clang_libcpp

00:02:38

passed #26255
build_gcc_49

00:02:14