[Build] NFC: Split BuildOperationBuildSystemDelegateHandler into LLBu… (#7667)

…ildCommands, LLBuildDescription and LLBuildProgressTracker

### Motivation:

There are 3 logically distinct things in this giant file which are
better kept separate.

### Modifications:

- Splits BuildOperationBuildSystemDelegateHandler.swift into 3 files -
one for commands, one for build description and one for progress
tracking.

### Result:

Concerns are properly separated.

(cherry picked from commit 8a4364c)

Author: xedin

Sources/Build/CMakeLists.txt

@@ -18,14 +18,16 @@ add_library(Build
   BuildManifest/LLBuildManifestBuilder+Product.swift
   BuildManifest/LLBuildManifestBuilder+Resources.swift
   BuildManifest/LLBuildManifestBuilder+Swift.swift
-  BuildOperationBuildSystemDelegateHandler.swift
   BuildOperation.swift
   BuildPlan/BuildPlan.swift
   BuildPlan/BuildPlan+Clang.swift
   BuildPlan/BuildPlan+Product.swift
   BuildPlan/BuildPlan+Swift.swift
   BuildPlan/BuildPlan+Test.swift
   ClangSupport.swift
+  LLBuildCommands.swift
+  LLBuildDescription.swift
+  LLBuildProgressTracker.swift
   SwiftCompilerOutputParser.swift
   TestObservation.swift)
 target_link_libraries(Build PUBLIC