[clang] [CIR] Add initial support for bitfields in structs (PR #142041)

[Andy Kaylor via cfe-commits]

================
@@ -223,21 +272,114 @@ void CIRRecordLowering::fillOutputFields() {
             fieldTypes.size() - 1;
       // A field without storage must be a bitfield.
       assert(!cir::MissingFeatures::bitfields());
+      if (!member.data)
+        setBitFieldInfo(member.fieldDecl, member.offset, fieldTypes.back());
     }
     assert(!cir::MissingFeatures::cxxSupport());
   }
 }
 
+void CIRRecordLowering::accumulateBitFields(
+    RecordDecl::field_iterator field, RecordDecl::field_iterator fieldEnd) {
+  // Run stores the first element of the current run of bitfields.  FieldEnd is
+  // used as a special value to note that we don't have a current run.  A
+  // bitfield run is a contiguous collection of bitfields that can be stored in
+  // the same storage block.  Zero-sized bitfields and bitfields that would
+  // cross an alignment boundary break a run and start a new one.
+  RecordDecl::field_iterator run = fieldEnd;
+  // Tail is the offset of the first bit off the end of the current run.  It's
+  // used to determine if the ASTRecordLayout is treating these two bitfields as
+  // contiguous.  StartBitOffset is offset of the beginning of the Run.
+  uint64_t startBitOffset, tail = 0;
+  assert(!cir::MissingFeatures::isDiscreteBitFieldABI());
+
+  // Check if OffsetInRecord (the size in bits of the current run) is better
----------------
andykaylor wrote:

```suggestion
  // Check if 'offsetInRecord' (the size in bits of the current run) is better
```

https://github.com/llvm/llvm-project/pull/142041