BPF: support type exist/size and enum exist/value relocations

Four new CO-RE relocations are introduced:
  - TYPE_EXISTENCE: whether a typedef/record/enum type exists
  - TYPE_SIZE: the size of a typedef/record/enum type
  - ENUM_VALUE_EXISTENCE: whether an enum value of an enum type exists
  - ENUM_VALUE: the enum value of an enum type

These additional relocations will make CO-RE bpf programs
more adaptive for potential kernel internal data structure
changes.

Differential Revision: https://reviews.llvm.org/D83878