Pull in original portion of patch

Remove C11 atomic path from atomic header. Update tests to check for specific errors. Other style fixes.

Further rebases

Rebase master

For those that are interested in testing out this change, here is a release build of clang 10 with these changes: https://skytopsoftware.ddns.net/binaries/compilers/llvm/llvm_10_release_linux_amd64.tar.gz

Any update on this?

Provide only two versions of __cxx_atomic_pointer_to_data that perform different cast operations. Get rid of type-specific versions by extracting original type qualifiers and re-annoting atomic type.

Remove redundant namespace qualifiers

Get rid of duplication across __cxx_atomic_* functions by using a traits class and SFINAE to check types and select overrides when required. Static assert that template type is always lock free when built using builtin only mode.