FunctionDepsFinder.cpp: Closure RecursiveASTVisitor do not respect vistor rules.

[giulianobelinassi]

From clang's documentation (llvm-project/clang/include/clang/AST/RecursiveASTVisitor.h):

/// A class that does preorder or postorder
/// depth-first traversal on the entire Clang AST and visits each node.
///
/// This class performs three distinct tasks:
///   1. traverse the AST (i.e. go to each node);
///   2. at a given node, walk up the class hierarchy, starting from
///      the node's dynamic type, until the top-most class (e.g. Stmt,
///      Decl, or Type) is reached.
///   3. given a (node, class) combination, where 'class' is some base
///      class of the dynamic type of 'node', call a user-overridable
///      function to actually visit the node.
///
/// These tasks are done by three groups of methods, respectively:
///   1. TraverseDecl(Decl *x) does task #1.  It is the entry point
///      for traversing an AST rooted at x.  This method simply
///      dispatches (i.e. forwards) to TraverseFoo(Foo *x) where Foo
///      is the dynamic type of *x, which calls WalkUpFromFoo(x) and
///      then recursively visits the child nodes of x.
///      TraverseStmt(Stmt *x) and TraverseType(QualType x) work
///      similarly.
///   2. WalkUpFromFoo(Foo *x) does task #2.  It does not try to visit
///      any child node of x.  Instead, it first calls WalkUpFromBar(x)
///      where Bar is the direct parent class of Foo (unless Foo has
///      no parent), and then calls VisitFoo(x) (see the next list item).
///   3. VisitFoo(Foo *x) does task #3.
///
/// These three method groups are tiered (Traverse* > WalkUpFrom* >
/// Visit*).  A method (e.g. Traverse*) may call methods from the same
/// tier (e.g. other Traverse*) or one tier lower (e.g. WalkUpFrom*).
/// It may not call methods from a higher tier.

This implies that the Visitor##DECL methods may not call Traverse##DECL methods, which may explain some of the weird behaviors we are seeing when analyzing kernel source. Refactor this so this partial order relation is respected.