[libc++][CI] Updates and improves the Docker image.

Since we branched LLVM install Clang 16 and remove Clang 12.

Currently our Docker installs 4 versions of Clang so our CI can use the
same image for both the main and the release branch. This wasn't done for
the other Clang tools so they always use the same version for testing
the main and the release branch. Instead install 2 versions for the
tools.

However it seems the default for Clang and its tools were the latest
released version instead of the ToT. To lessen the risk of breaking the
release CI, version 14 is installed hard-coded as a temporary solution.

Updating the main branch to use the Clang 16 compiler will be done in a
separate patch.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D131324