Fixes a couple of compiler warnings that occur frequently when building the compiler:
- Copy the nullability annotation definitions from `Visibility.h` to `BridgedSwiftObject.h` and wrap all code that contains nullability annotations in `SWIFT_BEGIN_NULLABILITY_ANNOTATIONS` and `SWIFT_END_NULLABILITY_ANNOTATIONS` (supressing the warning `type nullability specifier '_Nullable' is a Clang extension [-Wnullability-extension]`)
- Suppress warnings about using `$` (mangling prefix) as an identifier using pragmas (supressing the warning `'$' in identifier [-Wdollar-in-identifier-extension]`)
- Change the macro condition of `SWIFT_NODISCARD` from `__cplusplus >= 201402l` (which checked for >= C++14) to `__cplusplus > 201402l`. This appears to have been a copy-paste error from `LLVM_NODISCARD` (supressing the warning `use of the 'nodiscard' attribute is a C++17 extension [-Wc++17-extensions]`)
Add the list of symbols from the swift-5.0 release and only use that symbols to refer to pre-specializations in a Onone build.
This ensures that future Onone executables can link to an old swift 5.0 libswiftSwiftOnoneSupport library.
rdar://problem/48924409
