properly name the constant memory exposed to llvm

[oli-obk]

Due to historic accidents and severe cases of laziness, all allocations created by constant evaluation are named byte_str. This naming does not improve the debuggability of LLVM-IR and is rarely correct.

It would be great if the name given to the allocation would either be something derived from the context of the allocation (like the name of the static/constant/function that the allocation belongs to), or in case such information cannot be derived, either produce no name or just data as a fallback.

Proposed order of steps:

1. rename byte_str to data and adjust tests accordingly
2. Pass down None::<String> in

   rust/src/librustc_codegen_llvm/mir/operand.rs

   Line 97 in dbd10f8

   pub fn from_const(bx: &Builder<'a, 'tcx>,

   and

   rust/src/librustc_codegen_llvm/mir/constant.rs

   Line 31 in dbd10f8

   pub fn primval_to_llvm(cx: &CodegenCx,

   and potentially the functions calling either of these two functions.
3. Replace those None::<String> with an actual value where such information is available.

[eddyb]

If we can pass no name to LLVM, we should just do that, to keep things simple for both us and LLVM.

[AstralSorceress]

I'd like to take this.