Frontend: support -I=path for sysroot expansion

From the GCC manpage:

  -I dir
    ... If dir begins with =, then the = will be replaced by the sysroot prefix;
    see --sysroot and -isysroot.

Add support to expand the `=` as a prefix of the include path with the sysroot
if specified.  `-isysroot` takes precedence over `--sysroot` as the normal
argument behaviour occurs.  The ordering of the `-isysroot` is relevant to the
path substituted.  If no `--sysroot=` or `-isysroot` option is present, the = is
not expanded.

Resolves PR26965!

llvm-svn: 268777