[clang][BPF] support type exist/size and enum exist/value relocations

This patch added the following additional compile-once
run-everywhere (CO-RE) relocations:
  - existence/size of typedef, struct/union or enum type
  - enum value and enum value existence

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

For existence/size relocations, the following two code patterns
are supported:
  1. uint32_t __builtin_preserve_type_info(*(<type> *)0, flag);
  2. <type> var;
     uint32_t __builtin_preserve_field_info(var, flag);
flag = 0 for existence relocation and flag = 1 for size relocation.

For enum value existence and enum value relocations, the following code
pattern is supported:
  uint64_t __builtin_preserve_enum_value(*(<enum_type> *)<enum_value>,
                                         flag);
flag = 0 means existence relocation and flag = 1 for enum value.
relocation. In the above <enum_type> can be an enum type or
a typedef to enum type. The <enum_value> needs to be an enumerator
value from the same enum type. The return type is uint64_t to
permit potential 64bit enumerator values.

Differential Revision: https://reviews.llvm.org/D83242

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