We've built up quite a few links directly to github within the code
base. We should instead use `llvm.org/PR<issue-number>` to link to bugs,
since that is resilient to the bug tracker changing in the future. This
is especially relevant for tests linking to bugs, since they will
probably be there for decades to come. A nice side effect is that these
links are significantly shorter than the GH links, making them much less
of an eyesore.
This patch also replaces a few links that linked to the old bugzilla
instance on llvm.org.
The new operator<=> is mapped onto the existing functions
__libcpp_thread_id_equal and __libcpp_thread_id_less. Introducing a
new __libcpp_thread_id_compare_three_way might lead to more efficient
code. Given that we can still introduce __libcpp_thread_id_compare_three_way
later, for this commit I opted to not break ABI. If requested, I will
add __libcpp_thread_id_compare_three_way in a follow-up commit.
Implements part of P1614R2 "The Mothership has Landed"
Differential Revision: https://reviews.llvm.org/D131362
