Merge remote-tracking branch 'origin/llvm12'
Happy LLVM 12 Release Day.
Please note that the llvm 12 tag does not include the latest commit in
the release/12.x branch, which is in fact a bug fix for a regression
that is causing a failure in Zig's test suite.
Zig master branch is tracking release/12.x, and will be enabling the
test that is fixed by that commit.