[LLDB][NativePDB] Fix several crashes when parsing debug info.

1. If array element type is a tag decl, complete it.
2. Fix few places where `asTag` should be used instead of `asClass()`.
3. Handle the case that `PdbAstBuilder::CreateFunctionDecl` return nullptr mainly due to an existing workaround (`m_cxx_record_map`).
4. `FindMembersSize` should never return error as this would cause early exiting in `CVTypeVisitor::visitFieldListMemberStream` and then cause assertion failure.
5. In some pdbs from C++ runtime libraries have S_LPROC32 followed directly by S_LOCAL and the local variable location is a S_DEFRANGE_FRAMEPOINTER_REL. There is no information about base frame register in this case, ignoring it by returning RegisterId::NONE.
6. Add a TODO when S_DEFRANGE_SUBFIELD_REGISTER describes the variable location of a pointer type. For now, just ignoring it if the variable is pointer.