[clang] [llvm] [DebugInfo] Emit DW_AT_bit_size for _BitInt types and others (PR #164372)

[Orlando Cazalet-Hyams via cfe-commits]

================
@@ -766,8 +766,17 @@ void DwarfUnit::constructTypeDIE(DIE &Buffer, const DIBasicType *BTy) {
     addUInt(Buffer, dwarf::DW_AT_encoding, dwarf::DW_FORM_data1,
             BTy->getEncoding());
 
-  uint64_t Size = BTy->getSizeInBits() >> 3;
-  addUInt(Buffer, dwarf::DW_AT_byte_size, std::nullopt, Size);
+  uint64_t SizeInBytes = divideCeil(BTy->getSizeInBits(), 8);
----------------
OCHyams wrote:

I think the missing data_bit_offset in this case is ok too - the bit I quoted continues on a few sentences later:

> The data bit offset attribute is the offset in bits from the beginning of the containing storage to the beginning of the value. Bits that are part of the offset are padding. **If this attribute is omitted a default data bit offset of zero is assumed.**

GCC gives us this DWARF:
```
0x000000a1:   DW_TAG_base_type
                DW_AT_byte_size (0x02)
                DW_AT_encoding  (DW_ATE_signed)
                DW_AT_bit_size  (0x0f)
                DW_AT_name      ("_BitInt(15)")
```

The only difference between that and my patched Clang is that our name is `_BitInt` and GCC's is `_BitInt(15)`. (I'm happy to change that? not sure what's best - I suppose there's value to users if the bit-count is showing up in types in the debugger)

https://github.com/llvm/llvm-project/pull/164372