[BPF] Skip modifiers for __builtin_btf_type_id() local type (#71094)

BPF upstream reported an inconsistent behavior w.r.t. BPF_TYPE_ID_LOCAL
vs. BPF_TYPE_ID_TARGET (or BPF_TYPE_ID_REMOTE in LLVM terminology).

For BPF_TYPE_ID_TARGET, all modifiers (like 'const' and 'volatile') are
ignored in the final type encoding. For example, for type
 'const struct foo', the eventually encoding in BTF relocation
is 'struct foo'. This faciliates libbpf to match corresponding kernel
types with considering any modifiers.

Currently behavior for BPF_TYPE_ID_LOCAL is different. It will encode
'const struct foo' in BTF relocation and such discrepancy confused users
([1]).

This patch fixed this discrepancy by making BPF_TYPE_ID_LOCAL BTF type
representation the sams as BPF_TYPE_ID_TARGET. This should have minimum
user impact since ultimately user wants to get a real time not a 'const'
type modifier.

The selftest builtin-btf-type-id-2.ll is used to test BPF_TYPE_ID_TARGET
with 'const' modifier. Adapt the same test for BPF_TYPE_ID_LOCAL. And
the below diff shows now both BPF_TYPE_ID_LOCAL and BPF_TYPE_ID_TARGET
produces the same type:

$ diff test/CodeGen/BPF/BTF/builtin-btf-type-id-2.ll
test/CodeGen/BPF/BTF/builtin-btf-type-id-local.ll
--- test/CodeGen/BPF/BTF/builtin-btf-type-id-2.ll 2023-07-30
16:58:20.657528310 -0700
+++ test/CodeGen/BPF/BTF/builtin-btf-type-id-local.ll 2023-11-02
10:23:25.356959008 -0700
  @@ -6,7 +6,7 @@
   ;     int a;
   ;   };
   ;   int test(void) {
  -;     return __builtin_btf_type_id(*(const struct s *)0, 1);
  +;     return __builtin_btf_type_id(*(const struct s *)0, 0);
   ;   }
   ; Compilation flag:
; clang -target bpf -O2 -g -S -emit-llvm -Xclang -disable-llvm-passes
test.c
  $

[1]
https://lore.kernel.org/bpf/CAN+4W8h3yDjkOLJPiuKVKTpj_08pBz8ke6vN=Lf8gcA=iYBM-g@mail.gmail.com/



Co-authored-by: Yonghong Song <yonghong.song@linux.dev>