Allow multi-member variants in DWARF (#139300)

Currently, each variant in the variant part of a structure type can only
contain a single member. This was sufficient for Rust, where each
variant is represented as its own type.

However, this isn't really enough for Ada, where a variant can have
multiple members.

This patch adds support for this scenario. This is done by allowing the
use of DW_TAG_variant by DICompositeType, and then changing the DWARF
generator to recognize when a DIDerivedType representing a variant holds
one of these. In this case, the fields from the DW_TAG_variant are
inlined into the variant, like so:
```
 <4><7d>: Abbrev Number: 9 (DW_TAG_variant)
    <7e>   DW_AT_discr_value : 74
 <5><7f>: Abbrev Number: 7 (DW_TAG_member)
    <80>   DW_AT_name        : (indirect string, offset: 0x43): field0
    <84>   DW_AT_type        : <0xa7>
    <88>   DW_AT_alignment   : 8
    <89>   DW_AT_data_member_location: 0
 <5><8a>: Abbrev Number: 7 (DW_TAG_member)
    <8b>   DW_AT_name        : (indirect string, offset: 0x4a): field1
    <8f>   DW_AT_type        : <0xa7>
    <93>   DW_AT_alignment   : 8
    <94>   DW_AT_data_member_location: 8
```
Note that the intermediate DIDerivedType is still needed in this
situation, because that is where the discriminants are stored.

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/lib/IR/DIBuilder.cpp b/llvm/lib/IR/DIBuilder.cpp
index d9cc49f..90da9f3 100644
--- a/llvm/lib/IR/DIBuilder.cpp
+++ b/llvm/lib/IR/DIBuilder.cpp
@@ -444,6 +444,19 @@ DIDerivedType *DIBuilder::createVariantMemberType(
       std::nullopt, std::nullopt, Flags, getConstantOrNull(Discriminant));
 }
 
+DIDerivedType *DIBuilder::createVariantMemberType(DIScope *Scope,
+                                                  DINodeArray Elements,
+                                                  Constant *Discriminant,
+                                                  DIType *Ty) {
+  auto *V = DICompositeType::get(VMContext, dwarf::DW_TAG_variant, {}, nullptr,
+                                 0, getNonCompileUnitScope(Scope), {}, 0, 0, 0,
+                                 DINode::FlagZero, Elements, 0, {}, nullptr);
+
+  trackIfUnresolved(V);
+  return createVariantMemberType(Scope, {}, nullptr, 0, 0, 0, 0, Discriminant,
+                                 DINode::FlagZero, V);
+}
+
 DIDerivedType *DIBuilder::createBitFieldMemberType(
     DIScope *Scope, StringRef Name, DIFile *File, unsigned LineNumber,
     uint64_t SizeInBits, uint64_t OffsetInBits, uint64_t StorageOffsetInBits,