rustc: mark references w/anonymous lifetime nocapture

Closes #6751
Address review
test/run-pass/out-of-stack: prevent tco

We really do *not* want TCO to kick in. If it does, we'll never blow the
stack, and never trigger the condition the test is checking for. To that end,
do a meaningless alloc that serves only to get a destructor to run. The
addition of nocapture/noalias seems to have let LLVM do more TCO, which
hurt this testcase.
auto merge of #13001 : cmr/rust/unnamed-lifetime-nocapture, r=cmr,nikomatsakis

Closes #6751