pw_polyfill: Split up polyfill include paths

- Sharing a public_overrides/ path between polyfilled C++ headers means
  that all of the headers are visible during compilation. Work around
  this by having each header at its own include path.
- Have pw_span explicitly depend on the polyfills it needs when building
  with C++14.
- Remove the <type_traits> polyfill, since it isn't needed.
- Remove polyfills of C++17 features from the Bazel and CMake builds.
- Prevent using polyfill headers of C++17 features in C++17 or newer.
- Update pwbug/602 to b/235237667.

Bug: b/235237667
Change-Id: I53ae37ff761d7a66618af8d3a4dea2ba068d4acc
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/97442
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Ted Pudlik <tpudlik@google.com>