This changes `PdbAstBuilder` to a language-neutral abstract interface
and moves all of its functionality to the `PdbAstBuilderClang` derived
class.
All Clang-specific methods with external callers are now public methods
on `PdbAstBuilderClang`. `TypeSystemClang` and `UdtRecordCompleter` use
`PdbAstBuilderClang` directly.
Did my best to clean up includes and unused methods.
RFC for context:
https://discourse.llvm.org/t/rfc-lldb-make-pdbastbuilder-language-agnostic/89117
