General: Make use of std::erase_if/std::erase where applicable

Just some trivial copies that can be eliminated or turned into moves.

Makes for consistency with the surrounding code.

These aren't necessarily cheap to copy, since a control qualifier will
have around 3 std::strings inside of it, so passing by value can churn
allocations a little bit.

std::regex has a relatively expensive constructor, and these are unchanging regexes.

We allocate in MakeSuppressor via `return unique_ptr(std::make_unique<...>(...).release(), InvokingDeleter{}`, so it wasn't properly getting freed.

StripWhitespace maintains old behavior

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.