Improve the error message for missing else clauses in if expressions
export *IntSlice in libcollections
auto merge of #18015 : jakub-/rust/issue-4201, r=pcwalton

Closes #4201.
auto merge of #18067 : mahkoh/rust/immutableintslice, r=alexcrichton

There is also a second commit that adds them to the prelude.
debuginfo: Gate all LLDB debuginfo tests on a minimum LLDB version being available
debuginfo: Make some path LLDB-related paths in the test runner absolute to help the build bots.
debuginfo: Enable LLDB test suite
debuginfo: Let LLDB tests run in parallel again since our min-supported version has no problems with that.