[BPF] fix typedef issue for offset relocation

Currently, the CO-RE offset relocation does not work
if any struct/union member or array element is a typedef.
For example,
  typedef const int arr_t[7];
  struct input {
      arr_t a;
  };
  func(...) {
       struct input *in = ...;
       ... __builtin_preserve_access_index(&in->a[1]) ...
  }
The BPF backend calculated default offset is 0 while
4 is the correct answer. Similar issues exist for struct/union
typedef's.

When getting struct/union member or array element type,
we should trace down to the type by skipping typedef
and qualifiers const/volatile as this is what clang did
to generate getelementptr instructions.
(const/volatile member type qualifiers are already
ignored by clang.)

This patch fixed this issue, for each access index,
skipping typedef and const/volatile/restrict BTF types.

Signed-off-by: Yonghong Song <yhs@fb.com>

Differential Revision: https://reviews.llvm.org/D65259

llvm-svn: 367062