[clang] Match -isysroot behaviour with system compiler on Darwin

See discussion in https://reviews.llvm.org/D89001

I updated test to match actual behavior of
/Applications/Xcode_14.3.1_14E300b.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang
after that modified upstream to match the test.

Differential Revision: https://reviews.llvm.org/D157283