[libc] Libc premerge test doesn't check fullbuild

[michaelrj-google]

Currently the libc premerge tests only check overlay mode, but not fullbuild. This leads to the CI missing problems in fullbuild-only functions.

Ideally we would test both fullbuild and overlay modes.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Michael Jones (michaelrj-google)

Currently the libc premerge tests only check overlay mode, but not fullbuild. This leads to the CI missing problems in fullbuild-only functions.

Ideally we would test both fullbuild and overlay modes.