Tests for dynamic regexes will now run during 'check-stage2'.

Before, tests for dynamic regexes ran during stage1 and tests for
native regexes ran during stage2. But the buildbots don't test stage1,
so now both dynamic and native tests are run during stage2.

Closes #13740.
rustc: Restrict the scope of a borrow on def_map

This addresses the ICE from #13763, but it does not allow the test to compile,
due to #13768. An alternate test was checked in in the meantime.

Closes #13763
auto merge of #13766 : BurntSushi/rust/regex-dynamic-tests, r=alexcrichton

Before, tests for dynamic regexes ran during stage1 and tests for
native regexes ran during stage2. But the buildbots don't test stage1,
so now both dynamic and native tests are run during stage2.

Closes #13740.
auto merge of #13769 : alexcrichton/rust/restrict-some-scopes, r=huonw

This addresses the ICE from #13763, but it does not allow the test to compile,
due to #13768. An alternate test was checked in in the meantime.

Closes #13763