Revert "[NFC] Improve FileSpec internal APIs and usage in preparation for...

Revert "[NFC] Improve FileSpec internal APIs and usage in preparation for adding caching of resolved/absolute." and follow-ups

This reverts commit 9429b67b.

It broke the build on Windows, see comments on https://reviews.llvm.org/D130309

It also reverts these follow-ups:

Revert "Fix buildbot breakage after https://reviews.llvm.org/D130309."
This reverts commit f959d815.

Revert "Fix buildbot breakage after https://reviews.llvm.org/D130309."
This reverts commit 0bbce7a4.

Revert "Cache the value for absolute path in FileSpec."
This reverts commit dabe8772.