[clang] [clang] Lower _BitInt(129+) to a different type in LLVM IR (PR #91364)

[John McCall via cfe-commits]

================
@@ -89,7 +89,7 @@ void CodeGenTypes::addRecordTypeName(const RecordDecl *RD,
 /// ConvertType in that it is used to convert to the memory representation for
 /// a type.  For example, the scalar representation for _Bool is i1, but the
 /// memory representation is usually i8 or i32, depending on the target.
----------------
rjmccall wrote:

```suggestion
/// memory representation is usually i8 or i32, depending on the target.
///
/// We generally assume that the alloc size of this type under the LLVM
/// data layout is the same as the size of the AST type.  The alignment
/// does not have to match: Clang should always use explicit alignments
/// and packed structs as necessary to produce the layout it needs.
/// But the size does need to be exactly right or else things like struct
/// layout will break.
```

https://github.com/llvm/llvm-project/pull/91364