[Github] Use building LLVM as perf-training for CI container (#80713)

This patch adjusts the build process for building the toolchain for the
CI container to perform more rigorous perf-training for PGO,
particularly building the entirety of LLVM as that is what showed the
best results while benchmarking. This patch also splits the job into two
stages to avoid timeouts due to the large increase in buildtime. There
are a couple other hacks added in here to make things work that we can
do away with eventually once we're able to run jobs like this on more
powerful self-hosted runners.