[AutoDiff][Gardening] Use `SILFunction::getSourceFile` instead of dup logic (#84709)
In 025902fa994d83a213f6765277605c8d2f83c247,
`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.