[C++23] [Modules] `-fprebuilt-module-path` doesn't work under `-std=c++2b`

[ChuanqiXu9]

It is easy to reproduce. Just change the language standard option to -std=c++2b -fprebuilt-module-path=.. Then the compiler would warning for:

warning: argument unused during compilation: '-fprebuilt-module-path=.' [-Wunused-command-line-argument]

[llvmbot]

@llvm/issue-subscribers-clang-driver

[llvmbot]

@llvm/issue-subscribers-clang-modules

[llvmbot]

@llvm/issue-subscribers-c-2b

[ChuanqiXu9]

This is a basic issue. This should be good to be back ported to clang15.x.

[ChuanqiXu9]

/cherry-pick 834a878

[llvmbot]

/branch llvm/llvm-project-release-prs/issue56803