Revert change to get the compilation once during ModifyAllReferencesAsync

This change caused incorrect behavior since the document ids can come from projects other than the source project (namely the test project).
This broken when semantic models were requested for test project documents from the source project compilation.

Author: Exanite

sources/SilkTouch/SilkTouch/Mods/LocationTransformation/LocationTransformationUtils.cs

@@ -45,12 +45,6 @@ public static async Task ModifyAllReferencesAsync(
         ];
 
         var originalSolution = sourceProject.Solution;
-        var compilation = await sourceProject.GetCompilationAsync(ct);
-        if (compilation == null)
-        {
-            return;
-        }
-
         var newDocuments = new ConcurrentDictionary<DocumentId, SyntaxNode>();
         var symbolSet = new HashSet<ISymbol>(symbols, SymbolEqualityComparer.Default);
         var baseRewriter = new LocationTransformationRewriter(symbolSet, transformers.ToList());
@@ -66,13 +60,12 @@ await Parallel.ForEachAsync(
                 }
 
                 var originalRoot = await originalDocument.GetSyntaxRootAsync(ct);
-                if (originalRoot == null)
+                var semanticModel = await originalDocument.GetSemanticModelAsync(ct);
+                if (originalRoot == null || semanticModel == null)
                 {
                     return;
                 }
 
-                var semanticModel = compilation.GetSemanticModel(originalRoot.SyntaxTree);
-
                 // Since this is multithreaded, each thread needs their own copy of the rewriter and transformers
                 var rewriter = baseRewriter.GetThreadSafeCopy();