Extend `DIBasicType` and `DISubroutineType` with additional field
`annotations`, e.g. as below:
```
!5 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed, annotations: !6)
!6 = !{!7}
!7 = !{!"btf:type_tag", !"tag1"}
```
The field would be used by BPF backend to generate DWARF attributes
corresponding to `btf_type_tag` type attributes, e.g.:
```
0x00000029: DW_TAG_base_type
DW_AT_name ("int")
DW_AT_encoding (DW_ATE_signed)
DW_AT_byte_size (0x04)
0x0000002d: DW_TAG_LLVM_annotation
DW_AT_name ("btf:type_tag")
DW_AT_const_value ("tag1")
```
Such DWARF entries would be used to generate BTF definitions by tools
like [pahole](https://github.com/acmel/dwarves).
Note: similar fields with similar purposes are already present in
DIDerivedType and DICompositeType.
Currently "btf_type_tag" attributes are represented in debug information
as 'annotations' fields in DIDerivedType with DW_TAG_pointer_type tag.
The annotation on a pointer corresponds to pointee having the attributes
in the final BTF.
The discussion in
[thread](https://lore.kernel.org/bpf/87r0w9jjoq.fsf@oracle.com/) came to
conclusion, that such annotations should apply to the annotated type
itself. Hence the necessity to extend `DIBasicType` & `DISubroutineType`
types with 'annotations' field to represent cases like below:
```
int __attribute__((btf_type_tag("foo"))) bar;
```
This was previously tracked as differential revision:
https://reviews.llvm.org/D143966
3ca17443ef