In #183172, I added support for reading `S_REGREL32_INDIR` records. This
adds support in LLDB. The record is emitted by MSVC for structured
bindings and in the compiler generated coroutine stubs.
It describes a location at `*(Register + Offset) + OffsetInUdt`.
Equivalent to
```
DW_OP_breg{reg} {Offset}
DW_OP_deref
DW_OP_plus_uconst {OffsetInUdt}
```
LLVM doesn't create this record - it only creates `S_LOCAL`s for local
variables. We'll probably need `S_DEFRANGE_REGISTER_REL_INDIR` for this
- should be simple to figure out the structure, but I haven't seen that
record emitted yet.
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
