[CIR] Always set base subobject type for C++ records (#194504)

For unions and final C++ classes, computeRecordLayout previously left
the base-subobject type as a default-constructed (null) cir::RecordType.
The non-virtual size always equals the size for those kinds of records,
so no separate ".base" variant is needed, but `BaseSubobjectType` itself
was never set to the complete type either.

That null leaked through
getCIRGenRecordLayout(RD).getBaseSubobjectCIRType(), which is called by
getStorageType(const CXXRecordDecl *) when laying out a
[[no_unique_address]] / potentially-overlapping field. The null
mlir::Type was then stored as MemberInfo::data and tripped a
SmallVector::back() !empty() assertion in
CIRRecordLowering::fillOutputFields, because a member with null data is
interpreted as a bitfield placeholder and reads fieldTypes.back() that
was never pushed.

Match classic CodeGen (CGRecordLayoutBuilder.cpp) and unconditionally
set baseTy = *ty for any CXXRecordDecl, only constructing a separate
".base" record when nv-size != size (which excludes unions and final
classes by construction).

Reproducer:
```
  union U { int a; long b; };
  struct S {
    [[no_unique_address]] U u;
    int x;
  } s;
```

This comes from a crash on the libc++ test-suite (~1,500 tests affected,
all crashing inside CIRRecordLowering::lower while laying out
__compressed_pair_padding-style structs in <string>).