[AutoDiff][Gardening] Use SILFunction::getSourceFile instead of dup logic (#84709)

In 025902f,
`SILFunction::getSourceFile()`
is introduced. This patch makes use of that to avoid duplicating the
logic in `static SourceFile &getSourceFile(SILFunction *f)` in
lib/SILOptimizer/Differentiation/ADContext.cpp.

Author: kovdan01

lib/SILOptimizer/Differentiation/ADContext.cpp

@@ -62,10 +62,8 @@ ADContext::ADContext(SILModuleTransform &transform)
 
 /// Get the source file for the given `SILFunction`.
 static SourceFile &getSourceFile(SILFunction *f) {
-  if (f->hasLocation())
-    if (auto *declContext = f->getLocation().getAsDeclContext())
-      if (auto *parentSourceFile = declContext->getParentSourceFile())
-        return *parentSourceFile;
+  if (SourceFile *file = f->getSourceFile())
+    return *file;
   for (auto *file : f->getModule().getSwiftModule()->getFiles())
     if (auto *sourceFile = dyn_cast<SourceFile>(file))
       return *sourceFile;