[AST] Ignore implicit nodes in IgnoreUnlessSpelledInSource mode

Update the ASTNodeTraverser to dump only nodes spelled in source.  There
are only a few which need to be handled, but Decl nodes for which
isImplicit() is true are handled together.

Update the RAV instances used in ASTMatchFinder to ignore the nodes too.
As with handling of template instantiations, it is necessary to allow
the RAV to process the implicit nodes because they need to be visitable
before the first traverse() matcher is encountered.  An exception to
this is in the MatchChildASTVisitor, because we sometimes wish to make a
node matchable but make its children not-matchable.  This is the case
for defaulted CXXMethodDecls for example.

Extend TransformerTests to illustrate the kinds of problems that can
arise when performing source code rewriting due to matching implicit
nodes.

This change accounts for handling nodes not spelled in source when using
direct matching of nodes, and when using the has() and hasDescendant()
matchers.  Other matchers such as
cxxRecordDecl(hasMethod(cxxMethodDecl())) still succeed for
compiler-generated methods for example after this change.  Updating the
implementations of hasMethod() and other matchers is for a follow-up
patch.

Differential Revision: https://reviews.llvm.org/D90982