Auto merge of #54993 - TimNN:pda-tdl, r=eddyb

Auto merge of #54993 - TimNN:pda-tdl, r=eddyb

Support for the program data address space option of LLVM's Target Datalayout

This was introduced recently (specifically, for AVR, cc @dylanmckay).

(I came up with this when attempting to run [avr-rust](https://github.com/avr-rust/rust) rebased on the latest [rust-lang](https://github.com/rust-lang/rust) commits. If this requires a different design, some additional discussions, or is not something to pursue right now, I'd be happy to close this PR).

Note that this somewhat overlaps with @DiamondLovesYou's #51576, I think, although the implementation here is significantly simpler: Since the address space applies to _all_ program data, we can just check the pointee's type whenever we create an LLVM pointer type. If it is a function we use the program data address space; if not we use the default address space.

cc @eddyb, who has been reviewing #51576

Ref: https://llvm.org/docs/LangRef.html#data-layout