[include-cleaner] Ignore builtin symbols in the WalkAST.

There is no need to add headers for builtin symbols.

Additionally, there is a bonus benefit which help eliminate some bugs -- builtin
functions are modeled as implicit FunctionDecls in the clang AST, which results in
them being treated as normal FunctionDecls in the implementation of the include-cleaner
(going through the path: ast-node -> decl -> source location -> header).
And, the source location of these built-in symbols' AST nodes is not precise (e.g. points to the first call site),
which leads to subtle behavior that inserts a header of the call site.

Differential Revision: https://reviews.llvm.org/D147213