Add support for sysroot-relative system header search paths (#82084)

Clang supported header searchpaths of the form `-I =/path`, relative to
the sysroot if one is passed, but did not implement that behavior for
`-iquote`, `-isystem`, or `-idirafter`.

This implements the `=` portion of the behavior implemented by GCC for
these flags described in
https://gcc.gnu.org/onlinedocs/gcc/Directory-Options.html.