[BPF] Handling type conversions correctly for CO-RE

With newly added debuginfo type
metadata for preserve_array_access_index() intrinsic,
this patch did the following two things:
 (1). checking validity before adding a new access index
      to the access chain.
 (2). calculating access byte offset in IR phase
      BPFAbstractMemberAccess instead of when BTF is emitted.

For (1), the metadata provided by all preserve_*_access_index()
intrinsics are used to check whether the to-be-added type
is a proper struct/union member or array element.

For (2), with all available metadata, calculating access byte
offset becomes easier in BPFAbstractMemberAccess IR phase.
This enables us to remove the unnecessary complexity in
BTFDebug.cpp.

New tests are added for
  . user explicit casting to array/structure/union
  . global variable (or its dereference) as the source of base
  . multi demensional arrays
  . array access given a base pointer
  . cases where we won't generate relocation if we cannot find
    type name.

Differential Revision: https://reviews.llvm.org/D65618

llvm-svn: 367735