[DebugInfo][BPF] Add 'annotations' field for DIBasicType & DISubroutineType (#91422)

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

1 files changed, 12 insertions, 4 deletions

diff --git a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
index 9102f3a..bc06c55 100644
--- a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
+++ b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
@@ -1527,7 +1527,7 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     break;
   }
   case bitc::METADATA_BASIC_TYPE: {
-    if (Record.size() < 6 || Record.size() > 7)
+    if (Record.size() < 6 || Record.size() > 8)
       return error("Invalid record");
 
     IsDistinct = Record[0];
@@ -1535,10 +1535,14 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
                                 ? static_cast<DINode::DIFlags>(Record[6])
                                 : DINode::FlagZero;
 
+    Metadata *Annotations = nullptr;
+    if (Record.size() > 7 && Record[7])
+      Annotations = getMDOrNull(Record[7]);
+
     MetadataList.assignValue(
         GET_OR_DISTINCT(DIBasicType,
                         (Context, Record[1], getMDString(Record[2]), Record[3],
-                         Record[4], Record[5], Flags)),
+                         Record[4], Record[5], Flags, Annotations)),
         NextMetadataNo);
     NextMetadataNo++;
     break;
@@ -1703,7 +1707,7 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     break;
   }
   case bitc::METADATA_SUBROUTINE_TYPE: {
-    if (Record.size() < 3 || Record.size() > 4)
+    if (Record.size() < 3 || Record.size() > 5)
       return error("Invalid record");
     bool IsOldTypeRefArray = Record[0] < 2;
     unsigned CC = (Record.size() > 3) ? Record[3] : 0;
@@ -1713,9 +1717,13 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     Metadata *Types = getMDOrNull(Record[2]);
     if (LLVM_UNLIKELY(IsOldTypeRefArray))
       Types = MetadataList.upgradeTypeRefArray(Types);
+    Metadata *Annotations = nullptr;
+    if (Record.size() > 4 && Record[4])
+      Annotations = getMDOrNull(Record[4]);
 
     MetadataList.assignValue(
-        GET_OR_DISTINCT(DISubroutineType, (Context, Flags, CC, Types)),
+        GET_OR_DISTINCT(DISubroutineType,
+                        (Context, Flags, CC, Types, Annotations)),
         NextMetadataNo);
     NextMetadataNo++;
     break;