Make PathFragment an abstract class.

There are now four concrete implementations: RelativeUnixPathFragment, AbsoluteUnixPathFragment, RelativeWindowsPathFragment, AbsoluteWindowsPathFragment.

Goals:
-Reduce memory usage of PathFragment on non-Windows platforms while maintaining existing semantics.
-Make a few simple performance improvements along the way.
-Add TODOs for a few more simple performance improvements.
-Open the way for reducing code complexity of PathFragment. All of the Windows-specific stuff ought not complicate the code so much.

Non goals:
-Make the entire codebase as pretty as possible wrt PathFragment & Windows.
-Make PathFragment usage more sane in general (e.g. change semantics to ban coexistence of Windows and Unix PathFragments).
-Optimize PathFragment as much as possible wrt memory or even in any other dimensions (e.g. gc churn, cpu).

To elaborate, the primary motivation is per-instance memory usage of PathFragment on Unix platforms:

Before this change
------------------
+UseCompressedOops --> 32 bytes per instance
-UseCompressedOops --> 40 bytes per instance

After this change
------------------
+UseCompressedOops --> 24 bytes per instance
-UseCompressedOops --> 32 bytes per instance

Since Bazel can retain lots of PathFragments, the memory savings of this CL are fairly large.

RELNOTES: None
PiperOrigin-RevId: 154052905

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