this PR fixes#91835.
For `DeclRefExpr` in lambda's function body, it will references to
original variable declaration in AST rather than `FieldDecl` for lambda
class, so it's needed to find the corresponding `FieldDecl` and bind
`DeclRefExpr`'s value to it.
This is already implemented for variables that are not in a structured
binding structure, so I extracted that part of the code so that it can
be used in the structured binding case.
b55dd8f607