Update clang regex for build 425

Regex borrowed from MacOS.clang_build_version.

Fixes #18755.
Don't try to install from git if git is too old

This came up in tigerbrew, where the install failed for a user with
git 1.6.x: https://github.com/mistydemeo/tigerbrew/issues/9

TODO Given that this left the user with a chowned /usr/local but no
functional homebrew install, there should still be better error
handling here.
Curl flags: <= 10.5

So I can be lazy and not change this downstream