BPF: report better error message for BTF_TYPE_ID_REMOTE relo failure

Matteo Croce reported a bpf backend fatal error in
https://github.com/llvm/llvm-project/issues/52779

A simplified case looks like:
  $ cat bug.c
  extern int do_smth(int);
  int test() {
    return __builtin_btf_type_id(*(typeof(do_smth) *)do_smth, 1);
  }
  $ clang -target bpf -O2 -g -c bug.c
  fatal error: error in backend: Empty type name for BTF_TYPE_ID_REMOTE reloc
  ...

The reason for the fatal error is that the relocation is against
a DISubroutineType like type 13 below:
  !10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
  !11 = !{}
  !12 = !DILocation(line: 3, column: 10, scope: !7)
  !13 = !DISubroutineType(types: !14)
  !14 = !{!10, !10}

The DISubroutineType doesn't have a name and there is no way for
downstream bpfloader/kernel to do proper relocation for it.

But we can improve error message to be more specific for this case.
The patch improved the error message to be:
  fatal error: error in backend: SubroutineType not supported for BTF_TYPE_ID_REMOTE reloc

Differential Revision: https://reviews.llvm.org/D116063