This patch adds bazel tests for llvm-libc.
Some math tests rely on the mpfr library. This is controlled via the --@llvm-project//libc:libc_math_mpfr flag. It can take three values:
- external (default) will build mpfr and gmp from source.
- system will use the system installed mpfr library.
- disable will skip tests relying on mpfr.
Based on your other comment, I think this could be removed now? But also there's https://reviews.llvm.org/D123481 to remove -Werror from the default config. Note, though, that the CI still builds with -Werror