<flat_map>: Fix new libcxx test failure due to argument type mismatch when calling ranges::remove_if from flat_map::erase_if

[vmichal]

The newly failing libcxx test std\containers\container.adaptors\flat.multimap\robust_against_nonbool.compile.pass.cpp contains a template lambda written in such a way that template argument deduction fails unless it is called precisely with flat_map::const_reference

[]<class T>(std::pair<const StrictComparable<T>&, const StrictComparable<T>&>) -> BooleanTestable const& { return yes; };

Since the predicate given to flat_map::erase_if is specified to be callable with flat_map::const_reference in [flat.map.erasure]/2, I think the libcxx test is standards compliant.

Adjust the code of flat_map::erase_if to satisfy the test.

[StephanTLavavej]

Thanks for figuring out how to fix this! 😻