[include-cleaner] Only ignore builtins without a header

Certain standard library functions (e.g. std::move) are also implemented
as builtins. This patch moves filtering logic to the symbol->header
mapping phase to rather generate these references without any providers
only when we don't have a mapping.
That way we can also map them to header names mentioned in the builtin
mappings.

Differential Revision: https://reviews.llvm.org/D147449