Use the higher level APIs on SourceManager that handle locations in
parent vs child buffers. This then allows us to fix `walkToDeclPre`
such that we don't set the found DeclContext unless the location is
actually within that decl (here the location may well be in a
separate buffer as we may have a replaced function body).
A call to `SanitizeExpr` has been incorrectly removed from
`typeCheckForCodeCompletion` by refactoring to use `ASTNode`.
It is still required because fallback calls could have partially
type-checked AST.
Resolves: https://github.com/apple/swift/issues/59315
Resolves: rdar://94619388
