[clangd] Add go-to-definition support for fields in offsetof expressions (#192953)

clangd had no handling for OffsetOfExpr in FindTarget.cpp, so
go-to-definition and find-references on field names inside
__builtin_offsetof(Type, field) produced no results.

Fix this by adding VisitOffsetOfExpr to both visitors:
- The TargetFinder visitor in add(const Stmt*) so that allTargetDecls()
resolves the referenced FieldDecl(s).
- The refInStmt visitor so that findExplicitReferences() emits a
ReferenceLoc per field component, enabling find-references and rename to
locate each field at its source position.

Each OffsetOfNode of kind Field is visited; Identifier nodes (unresolved
dependent-type designators) and Base nodes (implicit base-class
indirections) are intentionally skipped.

Repro:
```                                                                                                                                                                                                                                             
  struct Foo { int bar; };                               
  int x = offsetof(Foo, bar);  // go-to-def on `bar` previously did nothing
```