-isysroot flag was added to all tests, but it makes
Driver/darwin-version.f90 failed.
In fact, only a few tests regarding interoperability with C need
-isysroot flag to search for headers and libraries. So, -isysroot flag
is now eliminated from the substitution `%flang`, and a new substitution
`%isysroot` has been introduced.
Moreover, Integration/iso-fortran-binding.cpp invokes clang++ via a
shell script, which makes it hard to add -isysroot flag. So, it is
refactored.
Fixes#150765
cc58ca5370