[BPF] handle derived type properly for computing type id

Currently, the type id for a derived type is computed incorrectly.
For example,
  type #1: int
  type #2: ptr to #1

For a global variable "int *a", type #1 will be attributed to variable "a".
This is due to a bug which assigns the type id of the basetype of
that derived type as the derived type's type id. This happens
to "const", "volatile", "restrict", "typedef" and "pointer" types.

This patch fixed this bug, fixed existing test cases and added
a new one focusing on pointers plus other derived types.

Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 356727