Rollup merge of #137257 - compiler-errors:fake-borrow-of-packed-field, r=oli-obk

Ignore fake borrows for packed field check

We should not emit unaligned packed field reference errors for the fake borrows that we generate during match lowering.

These fake borrows are there to ensure in *borrow-checking* that we don't modify the value being matched (which is why this only occurs when there's a match guard, in this case `if true`), but they are removed after the MIR is processed by `CleanupPostBorrowck`, since they're really just there to cause borrowck errors if necessary.

I modified `PlaceContext::is_borrow` since that's used by the packed field check:
https://github.com/rust-lang/rust/blob/17c1c329a5512d718b67ef6797538b154016cd34/compiler/rustc_mir_transform/src/check_packed_ref.rs#L40

It's only used in one other place, in the SROA optimization (by which fake borrows are removed, so it doesn't matter):
https://github.com/rust-lang/rust/blob/17c1c329a5512d718b67ef6797538b154016cd34/compiler/rustc_mir_dataflow/src/value_analysis.rs#L922

Fixes https://github.com/rust-lang/rust/issues/137250

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