[BPF] Fix a BTF gen bug related to a pointer struct member

Currently, BTF generation stops at pointer struct members
if the pointee type is a struct. This is to avoid bloating
generated BTF size. The following is the process to
correctly record types for these pointee struct types.
  - During type traversal stage, when a struct member, which
    is a pointer to another struct, is encountered,
    the pointee struct type, keyed with its name, is
    remembered in a Fixup map.
  - Later, when all type traversal is done, the Fixup map
    is scanned, based on struct name matching, to either
    resolve as pointing to a real already generated type
    or as a forward declaration.

Andrii discovered a bug if the struct member pointee struct
is anonymous. In this case, a struct with empty name is
recorded in Fixup map, and later it happens another anonymous
struct with empty name is defined in BTF. So wrong type
resolution happens.

To fix the problem, if the struct member pointee struct
is anonymous, pointee struct type will be generated in
stead of being put in Fixup map.

Differential Revision: https://reviews.llvm.org/D82976