[clang] [clang] Ensure `--print-runtime-dir` path exists (PR #102834)

[Fangrui Song via cfe-commits]

MaskRay wrote:

`--print-runtime-dir` can be defined in multiple ways.
The current definition is to print a directory suitable for `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=on` configurations.
This interppretation makes sense to me, as the "runtime" in "print-runtime-dir" matches `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR`.

With this interpretation, `--print-runtime-dir` should print the directory regardless of whether the `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=on` directory is present or not.
If the directory is absent, `--print-runtime-dir` prints a directory that the user should ensure presence.


https://github.com/llvm/llvm-project/pull/102834