[LLDB][NativePDB] NFC: Add language-agnostic interface to PdbAstBuilder (#173111)

This change implements part of
https://discourse.llvm.org/t/rfc-lldb-make-pdbastbuilder-language-agnostic/89117,
the goal of which is make `PdbAstBuilder` language-agnostic.

It adds several new interface methods to `PdbAstBuilder` which allow
`SymbolFileNativePDB` to interact with it without using Clang-specific
types. These new methods would make up much of the interface of a future
abstract `PdbAstBuilder`. `SymbolFileNativePDB` is updated to use these.

The old Clang-specific methods were renamed if used by
`UdtRecordCompleter` or internally, or removed where possible.

GitOrigin-RevId: 39dd1fd5e163dd9c3f626377ed55faecd592f803