[Clang][BPF] implement __builtin_btf_type_id() builtin function

Such a builtin function is mostly useful to preserve btf type id
for non-global data. For example,
   extern void foo(..., void *data, int size);
   int test(...) {
     struct t { int a; int b; int c; } d;
     d.a = ...; d.b = ...; d.c = ...;
     foo(..., &d, sizeof(d));
   }

The function "foo" in the above only see raw data and does not
know what type of the data is. In certain cases, e.g., logging,
the additional type information will help pretty print.

This patch implemented a BPF specific builtin
  u32 btf_type_id = __builtin_btf_type_id(param, flag)
which will return a btf type id for the "param".
flag == 0 will indicate a BTF local relocation,
which means btf type_id only adjusted when bpf program BTF changes.
flag == 1 will indicate a BTF remote relocation,
which means btf type_id is adjusted against linux kernel or
future other entities.

Differential Revision: https://reviews.llvm.org/D74668