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, 1 insertions, 0 deletions

diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index 83c1264..2cfd382 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -1343,6 +1343,7 @@ void Verifier::visitDICompositeType(const DICompositeType &N) {
               N.getTag() == dwarf::DW_TAG_enumeration_type ||
               N.getTag() == dwarf::DW_TAG_class_type ||
               N.getTag() == dwarf::DW_TAG_variant_part ||
+              N.getTag() == dwarf::DW_TAG_variant ||
               N.getTag() == dwarf::DW_TAG_namelist,
           "invalid tag", &N);