[lldb] Fix Type::GetByteSize for pointer types

The function was returning an incorrect (empty) value on the first
invocation. Given that this only affected the first invocation, this
bug/typo went mostly unaffected. DW_AT_const_value were particularly
badly affected by this as the GetByteSize call is
SymbolFileDWARF::ParseVariableDIE is likely to be the first call of this
function, and its effects cannot be undone by retrying.

Depends on D86348.

Differential Revision: https://reviews.llvm.org/D86436