[LLDB][NativePDB] Let native pdb use class layout in debug info.

[LLDB][NativePDB] Let native pdb use class layout in debug info.

Before, class layout in native pdb is not hooked up in [[https://github.com/llvm/llvm-project/blob/main/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp#L9375-L9380 | here]].
This changes hooked it up by refactoring SymbolFileNativePDB and PdbAstBuilder.
PdbIndex (corresponds to a single pdb file) is removed from PdbAstBuilder, so it
can only be accessed via SymbolFileNativePDB and we can construct PdbAstBuilder
with just TypeSystemClang.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D134509