[libc++] NFC: Fix several GCC warnings in the test suite

- Several -Wshadow warnings
- Several places where we did not initialize our base class explicitly
- Unused variable warnings
- Some tautological comparisons
- Some places where we'd pass null arguments to functions expecting
  non-null (in unevaluated contexts)
- Add a few pragmas to turn off spurious warnings
- Fix warnings about declarations that don't declare anything
- Properly disable deprecation warnings in ext/ tests (the pragmas we
  were using didn't work on GCC)
- Disable include_as_c.sh.cpp because GCC complains about C++ flags
  when compiling as C. I couldn't find a way to fix this one properly,
  so I'm disabling the test. This isn't great, but at least we'll be
  able to enable warnings in the whole test suite with GCC.