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

This reverts commit 9429b67b8e.

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 f959d815f4.

Revert "Fix buildbot breakage after https://reviews.llvm.org/D130309."
This reverts commit 0bbce7a4c2.

Revert "Cache the value for absolute path in FileSpec."
This reverts commit dabe877248.

lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp

@@ -241,7 +241,7 @@ void ScriptInterpreterPython::ComputePythonDir(
   llvm::sys::path::append(path, LLDB_PYTHON_RELATIVE_LIBDIR);
 
 #if defined(_WIN32)
-  // This will be injected directly through FileSpec.SetDirectory(),
+  // This will be injected directly through FileSpec.GetDirectory().SetString(),
   // so we need to normalize manually.
   std::replace(path.begin(), path.end(), '\\', '/');
 #endif
@@ -260,7 +260,7 @@ FileSpec ScriptInterpreterPython::GetPythonDir() {
 #else
     ComputePythonDir(path);
 #endif
-    spec.SetDirectory(path);
+    spec.GetDirectory().SetString(path);
     return spec;
   }();
   return g_spec;