Switch Bazel premerge checks to use llvm-bazel-premerge queue
Switch Bazel premerge checks to use llvm-bazel-premerge queue

I created a new queue for the pre-merge checks so that they won't compete for resources
with the post-merge build already running. Obviously that creates some inefficiency, but is
a simple way to make sure neither build gets starved (in particular, we know that the
current setup of 4 agents running on a single 32-core VM is enough to handle the commit
traffic on the main branch whereas pre-merge testing volume is likely to be more
unpredictable.
google/llvm-premerge-checks,google/llvm-premerge-checks