[clang][Driver] allow tilde in user config dir

This patch allows users to configure clang with option
e.g. `-DCLANG_CONFIG_FILE_USER_DIR=~/.config/clang` or invoke clang
with `--config-user-dir=~/.config/clang`.

Patch merged on behalf of @paperchalice (LJC)

Differential Revision: https://reviews.llvm.org/D136940