Build the first LLVM without LTO in try builds
Currently, we perform three LLVM builds in the Linux x64 dist builder, which is used for `try` builds:
1) "Normal" LLVM - takes ~5s to compile thanks to `sccache`, but ~8 minutes to link because of ThinLTO
2) PGO instrumented LLVM - same timings as 1)
3) PGO optimized LLVM - takes about 20 minutes to build
When I tried to disable LTO for build 1), it suddenly takes only about a minute to build, because the linking step is much faster. The first LLVM doesn't really need LTO all that much. Without it, it will be a bit slower to build `rustc` in two subsequent steps, but it seems that the ~7 minutes saved on linking it do win that back.
Btw, we can't use the host LLVM for build 1), because this LLVM then builds `rustc` in PGO instrumented mode, and we need the same compiler when later PGO optimizing `rustc`. And we want to use our in-house LLVM for that I think.
