selftests/bpf: fix btf_dump/btf_dump due to recent clang change

selftests/bpf: fix btf_dump/btf_dump due to recent clang change

BugLink: https://bugs.launchpad.net/bugs/1981864
[ Upstream commit 4050764cbaa25760aab40857f723393c07898474 ]

Latest llvm-project upstream had a change of behavior
related to qualifiers on function return type ([1]).
This caused selftests btf_dump/btf_dump failure.
The following example shows what changed.

  $ cat t.c
  typedef const char * const (* const (* const fn_ptr_arr2_t[5])())(char * (*)(int));
  struct t {
    int a;
    fn_ptr_arr2_t l;
  };
  int foo(struct t *arg) {
    return arg->a;
  }

Compiled with latest upstream llvm15,
  $ clang -O2 -g -target bpf -S -emit-llvm t.c
The related generated debuginfo IR looks like:
  !16 = !DIDerivedType(tag: DW_TAG_typedef, name: "fn_ptr_arr2_t", file: !1, line: 1, baseType: !17)
  !17 = !DICompositeType(tag: DW_TAG_array_type, baseType: !18, size: 320, elements: !32)
  !18 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !19)
  !19 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !20, size: 64)
  !20 = !DISubroutineType(types: !21)
  !21 = !{!22, null}
  !22 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !23, size: 64)
  !23 = !DISubroutineType(types: !24)
  !24 = !{!25, !28}
  !25 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !26, size: 64)
  !26 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !27)
  !27 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
You can see two intermediate const qualifier to pointer are dropped in debuginfo IR.

With llvm14, we have following debuginfo IR:
  !16 = !DIDerivedType(tag: DW_TAG_typedef, name: "fn_ptr_arr2_t", file: !1, line: 1, baseType: !17)
  !17 = !DICompositeType(tag: DW_TAG_array_type, baseType: !18, size: 320, elements: !34)
  !18 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !19)
  !19 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !20, size: 64)
  !20 = !DISubroutineType(types: !21)
  !21 = !{!22, null}
  !22 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !23)
  !23 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !24, size: 64)
  !24 = !DISubroutineType(types: !25)
  !25 = !{!26, !30}
  !26 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !27)
  !27 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !28, size: 64)
  !28 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !29)
  !29 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
All const qualifiers are preserved.

To adapt the selftest to both old and new llvm, this patch removed
the intermediate const qualifier in const-to-ptr types, to make the
test succeed again.

  [1] https://reviews.llvm.org/D125919

Reported-by: Mykola Lysenko <mykolal@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220523152044.3905809-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>