[llvm] MCAsmBackend::applyFixup: Change `Data` to indicate the relocated location (PR #151724)

[Sam Elliott via llvm-commits]

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@@ -123,8 +123,8 @@ class LLVM_ABI MCAsmBackend {
   /// appropriate. Errors (such as an out of range fixup value) should be
   /// reported via \p Ctx.
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lenary wrote:

Please add some documentation to this function to explain that the bounds on `Data` are given by the combination of fragment size and fixup offset. This is not obvious from the signature.

https://github.com/llvm/llvm-project/pull/151724